Method and device for transmitting reference signal in wireless communication system supporting multiple antennas

ABSTRACT

The present invention relates to a method and device for transmitting a reference signal at a transmission end in a wireless communication system supporting multiple antennas. In particular, the present invention includes a step of transmitting a first reference signal related to pre-coded horizontal antenna domains and a second reference signal related to pre-coded vertical antenna domains to a reception end, wherein at least one of the pre-coded horizontal antenna domains is included in the vertical antenna domains.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/Kr2014/005952, filed on Jul. 3, 2014, which claims priorityunder 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/843,039,filed on Jul. 4, 2013, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting areference signal in a wireless communication system supporting multipleantennas.

BACKGROUND ART

MIMO (multi-input multi-output) technology means a method of improvingdata transceiving efficiency by adopting multiple transmitting antennasand multiple receiving antennas instead of a single transmitting antennaand a single receiving antenna. In particular, this technology increasescapacity or enhances performance using multiple antennas in atransmitting or receiving end of a wireless communication system. ThisMIMO technology may be called multi-antenna technology.

In order to support MIMO transmission, it may be able to use a precodingmatrix to appropriately distribute transmission information to eachantenna in accordance with a channel status and the like. In theconventional 3GPP (3^(rd) generation partnership project) LTE (long termevolution) system, maximum 4 transmitting antennas are supported fordownlink transmission and a corresponding precoding codebook is defined.

DISCLOSURE OF THE INVENTION Technical Task

Based on the above-described discussion, a method and apparatus fortransmitting a reference signal in a wireless communication systemsupporting multiple antennas will hereinafter be proposed.

Technical tasks obtainable from the present invention are non-limited bythe above-mentioned technical task. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

Technical Solutions

In a 1^(st) technical aspect of the present invention, provided hereinis a method of transmitting a reference signal, which is transmitted bya transmitting end in a wireless communication system supportingmultiple antennas, including the step of transmitting a 1^(st) referencesignal with respect to precoded horizontal antenna domains and a 2^(nd)reference signal with respect to precoded vertical antenna domains to areceiving end, wherein at least one specific antenna domain among theprecoded horizontal antenna domains is included in the precoded verticalantenna domains.

Preferably, the 2^(nd) reference signal may be generated based on theprecoded vertical antenna domains except the specific antenna domain.

Preferably, the 1^(st) reference signal and the 2^(nd) reference signalmay be used by the receiving end to restore entire channels of themultiple antennas through Kronecker product.

Preferably, the reference signal may correspond to a demodulationreference signal (DM-RS).

Preferably, the specific antenna domain may be configured through RRC(radio resource control) signaling.

Preferably, the method of transmitting the reference signal may furtherinclude the step of transmitting downlink control information (DCI)including information on the precoded horizontal antenna domains andinformation on the precoded vertical antenna domains. More preferably,the DCI may further include a bit index indicating a reference signalscheme in the transmitting end.

Preferably, the transmitting step may include the step of transmitting a3^(rd) reference signal including the 1^(st) reference signal and the2^(nd) reference signal.

In a 2^(nd) technical aspect of the present invention, provided hereinis a transmitter for transmitting a reference signal in a wirelesscommunication system supporting multiple antennas, including a radiofrequency unit and a processor configured to transmit a 1^(st) referencesignal with respect to precoded horizontal antenna domains and a 2^(nd)reference signal with respect to precoded vertical antenna domains to areceiving end, wherein at least one specific antenna domain among theprecoded horizontal antenna domains is included in the precoded verticalantenna domains.

Preferably, the 2^(nd) reference signal may be generated based on theprecoded vertical antenna domains except the specific antenna domain.

Advantageous Effects

According to an embodiment of the present invention, a reference signalcan be efficiently transmitted in a wireless communication systemsupporting multiple antennas.

Effects obtainable from the present invention are non-limited by theabove mentioned effect. And, other unmentioned effects can be clearlyunderstood from the following description by those having ordinary skillin the technical field to which the present invention pertains.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a diagram to describe a structure of a downlink radio frame.

FIG. 2 is a diagram for one example of a resource grid for one downlinkslot.

FIG. 3 is a diagram for a structure of a downlink subframe.

FIG. 4 is a diagram for a structure of an uplink subframe.

FIG. 5 is a diagram for a pattern of a common reference signal (CRS).

FIG. 6 is a diagram to describe a shift of a reference signal pattern.

FIG. 7 and FIG. 8 are diagrams to describe a resource element group(REG) corresponding to a unit to which downlink control channels areassigned.

FIG. 9 is a diagram to illustrate a scheme of transmitting PCFICH.

FIG. 10 is a diagram to illustrate locations of PCFICH and PERCH.

FIG. 11 is a diagram to illustrate a location of a downlink resourceelement to which a PHICH group is mapped.

FIG. 12 is a diagram for a structure of a transmitter according to anSC-FDMA scheme

FIG. 13 is a diagram to describe a scheme of mapping a DFT-processedsignal into a frequency domain.

FIG. 14 is a block diagram to describe a process for transmission of areference signal.

FIG. 15 is a diagram for a location of a symbol to which a referencesignal is mapped.

FIGS. 16 to 19 are diagrams to describe a clustered DFT-s-OFDMA scheme.

FIG. 20 is a diagram for a structure of an MIMO system.

FIG. 21 is a block diagram to describe functionality of an MIMO system.

FIG. 22 is a diagram to describe a basic concept of codebook basedprecoding.

FIG. 23 is a diagram for examples of configuration of 8 transmittingantennas.

FIG. 24 is a reference diagram of a 2-dimensional active antenna systemaccording to the present invention.

FIG. 25 is a reference diagram to describe an embodiment of the presentinvention.

FIG. 26 is a diagram for configurations of a base station device and auser equipment device according to the present invention.

BEST MODE FOR INVENTION

First of all, the following embodiments correspond to combinations ofelements and features of the present invention in prescribed forms. And,the respective elements or features may be considered as selectiveunless they are explicitly mentioned. Each of the elements or featurescan be implemented in a form failing to be combined with other elementsor features. Moreover, an embodiment of the present invention may beimplemented by combining elements and/or features together in part. Asequence of operations explained for each embodiment of the presentinvention may be modifiable. Some configurations or features of oneembodiment may be included in another embodiment or substituted withcorresponding configurations or features of another embodiment.

In this specification, embodiments of the present invention aredescribed centering on the data transmission/reception relations betweena base station and a terminal. In this case, the base station may bemeaningful as a terminal node of a network which directly performscommunication with the terminal. In this disclosure, a specificoperation explained as performed by a base station may be performed byan upper node of the base station in some cases.

In particular, in a network constructed with a plurality of networknodes including a base station, it is apparent that various operationsperformed for communication with a terminal can be performed by a basestation or other networks except the base station. ‘Base station (BS)’may be substituted with such a terminology as a fixed station, a Node B,an eNode B (eNB), an access point (AP) and the like. Moreover, in thisspecification, a terminology called a base station may be conceptionallyused as including a cell or a sector. Meanwhile, a relay may besubstituted with such a terminology as a relay node (RN), a relaystation (RS) and the like. And, ‘terminal’ may be substituted with sucha terminology as a user equipment (UE), a mobile station (MS), a mobilesubscriber station (MSS), a subscriber station (SS) and the like. Inthis specification, an uplink transmission entity may mean a terminal ora relay. And, an uplink reception entity may mean a base station or arelay. Similarly, a downlink transmission entity may mean a base stationor a relay. And, a downlink reception entity may mean a terminal or arelay. So to speak, an uplink transmission may mean a transmission froma terminal to a base station, a transmission from a terminal to a relay,or a transmission from a relay to a base station. Similarly, a downlinktransmission may mean a transmission from a base station to a terminal,a transmission from a base station to a relay, or a transmission from arelay to a terminal.

Specific terminologies used for the following description may beprovided to help the understanding of the present invention. And, theuse of the specific terminology may be modified into other forms withinthe scope of the technical idea of the present invention.

Occasionally, to prevent the present invention from getting vaguer,structures and/or devices known to the public may be skipped orrepresented as block diagrams centering on the core functions of thestructures and/or devices. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like partsin this specification.

Embodiments of the present invention may be supported by the disclosedstandard documents of at least one of wireless access systems includingIEEE 802 system, 3GPP system, 3GPP LTE and LTE-A (LTE-Advanced) systemand 3GPP2 system. In particular, the steps or parts, which are notexplained to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. Moreover, all terminologies disclosed in this documentmay be supported by the above standard documents.

The following description of embodiments of the present invention mayapply to various wireless access systems including CDMA (code divisionmultiple access), FDMA (frequency division multiple access), TDMA (timedivision multiple access), OFDMA (orthogonal frequency division multipleaccess), SC-FDMA (single carrier frequency division multiple access) andthe like. CDMA can be implemented with such a radio technology as UTRA(universal terrestrial radio access), CDMA 2000 and the like. TDMA canbe implemented with such a radio technology as GSM/GPRS/EDGE (GlobalSystem for Mobile communications)/General Packet Radio Service/EnhancedData Rates for GSM Evolution). OFDMA can be implemented with such aradio technology as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, E-UTRA (Evolved UTRA), etc. UTRA is a part of UMTS (UniversalMobile Telecommunications System). 3GPP (3rd Generation PartnershipProject) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS)that uses E-UTRA. The 3GPP LTE adopts OFDMA in downlink (hereinafterabbreviated) DL and SC-FDMA in uplink (hereinafter abbreviated UL). And,LTE-A (LTE-Advanced) is an evolved version of 3GPP LTE. WiMAX may beexplained by IEEE 802.16e standard (e.g., WirelessMAN-OFDMA referencesystem) and advanced IEEE 802.16m standard (e.g., WirelessMAN-OFDMAadvanced system). For clarity, the following description mainly concerns3GPP LTE system or 3GPP LTE-A system, by which the technical idea of thepresent invention may be non-limited.

A structure of a downlink (DL) radio frame is described with referenceto FIG. 1 as follows.

In a cellular OFDM radio packet communication system, UL/DL(uplink/downlink) data packet transmission is performed by a unit ofsubframe. And, one subframe is defined as a predetermined time intervalincluding a plurality of OFDM symbols. In the 3GPP LTE standard, atype-1 radio frame structure applicable to FDD (frequency divisionduplex) and a type-2 radio frame structure applicable to TDD (timedivision duplex) are supported.

FIG. 1 (a) is a diagram for a structure of a downlink radio frame oftype 1. A DL (downlink) radio frame includes 10 subframes. Each of thesubframes includes 2 slots. And, a time taken to transmit one subframeis defined as a transmission time interval (hereinafter abbreviatedIII). For instance, one subframe may have a length of 1 ms and one slotmay have a length of 0.5 ms. One slot may include a plurality of OFDMsymbols in time domain or may include a plurality of resource blocks(RBs) in frequency domain. Since 3GPP system uses OFDMA in downlink,OFDM symbol indicates one symbol duration. The OFDM symbol may be namedSC-FDMA symbol or symbol duration. Resource block (RB) is a resourceallocation unit and may include a plurality of contiguous subcarriers inone slot.

The number of OFDM symbols included in one slot may vary in accordancewith a configuration of CP. The CP may be categorized into an extendedCP and a normal CP. For instance, in case that OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be 7. In case that OFDM symbols are configured by the extendedCP, since a length of one OFDM symbol increases, the number of OFDMsymbols included in one slot may be smaller than that of the case of thenormal CP. In case of the extended CP, for instance, the number of OFDMsymbols included in one slot may be 6. If a channel status is unstable(e.g., a UE is moving at high speed), it may be able to use the extendedCP to further reduce the inter-symbol interference.

When a normal CP is used, since one slot includes 7 OFDM symbols, onesubframe includes 14 OFDM symbols. In this case, first 2 or 3 OFDMsymbols of each subframe may be allocated to PDCCH (physical downlinkcontrol channel), while the rest of the OFDM symbols are allocated toPDSCH (physical downlink shared channel).

FIG. 1 (b) is a diagram for a structure of a downlink radio frame oftype 2. A type-2 radio frame includes 2 half frames. Each of the halfframe includes 5 subframes, DwPTS (downlink pilot time slot), GP (guardperiod) and UpPTS (uplink pilot time slot). And, one of the subframesincludes 2 slots. The DwPTS is used for initial cell search,synchronization or channel estimation in a user equipment. The UpPTS isused for channel estimation in a base station and uplink transmissionsynchronization of a user equipment. The guard period is a period foreliminating interference generated in uplink due to multi-path delay ofa downlink signal between uplink and downlink. Meanwhile, 1 subframe isconstructed with 2 slots irrespective of a type of a radio frame.

The above-described structures of the radio frame are just exemplary.And, the number of subframes included in a radio frame, the number ofslots included in the subframe and the number of symbols included in theslot may be modified in various ways.

FIG. 2 is a diagram for one example of a resource grid for a downlink(DL) slot. This corresponds to a case that OFDM symbol includes a normalCP. Referring to FIG. 2, a downlink slot includes a plurality of OFDMsymbols in time domain and includes a plurality of resource blocks infrequency domain. In this case, for example, a single downlink slotincludes 7 OFDM symbols and a single resource block includes 12subcarriers, by which configurations of the downlink slot and theresource block are non-limited. Each element on a resource grid iscalled a resource element (RE). for instance, a resource element a (k,l) becomes a resource element located at k^(th) subcarrier and l^(th)OFDM symbol. In case of a normal CP, a single resource block includes12×7 resource elements [in case of an extended CP, 12×6 resourceelements are included]. Since an interval of each subcarrier is 15 kHz,a single resource block includes about 180 kHz in frequency domain.N^(DL) indicates the number of resource blocks included in a downlinkslot. And, the value of N^(DL) may be determined depending on a downlinktransmission bandwidth set up by scheduling of a base station.

FIG. 3 is a diagram for a structure of a downlink (DL) subframe. Maximum3 OFDM symbols situated in a head part of a first slot of one subframecorrespond to a control region to which a control channel is allocated.The rest of OFDM symbols correspond to a data region to which PDSCH(physical downlink shared channel) is allocated. A basic unit oftransmission becomes one subframe. In particular, PDCCH and PDSCH areassigned across 2 slots. Examples of DL control channels used by 3GPPLTE system may include PCFICH (Physical Control Format IndicatorChannel), PDCCH (Physical Downlink Control Channel), PHICH (Physicalhybrid automatic repeat request indicator Channel) and the like. ThePCFICH is transmitted in a first OFDM symbol of a subframe and includesinformation on the number of OFDM symbols used for a transmission of acontrol channel within the subframe. The PHICH includes HARQ ACK/NACKsignal in response to a UL transmission. Control information carried onPDCCH may be called downlink control information (DCI). The DCI mayinclude UL or DL scheduling information or a UL transmission powercontrol command for a random UE (user equipment) group. The PDCCH mayinclude transmission format and resource allocation information ofDL-SCH (downlink shared channel), resource allocation information onUL-SCH (uplink shared channel), paging information on PCH (pagingchannel), system information on DL-SCH, resource allocation of such ahigher layer control message as a random access response transmitted onPDSCH, transmission power control command set for individual UEs withina random UE group, transmission power control information, activation ofVoIP (voice over IP) and the like. A plurality of PDCCHs can betransmitted within the control region. A user equipment may be able tomonitor a plurality of the PDCCHs. The PDCCH is transmitted as anaggregation of at least one or more contiguous CCEs (control channelelements). The CCE is a logical allocation unit used to provide thePDCCH at a coding rate based on a radio channel status. The CCE maycorrespond to a plurality of REGs (resource element groups). A format ofthe PDCCH and the number of available PDCCH bits may be determined inaccordance with correlation between the number of CCEs and a coding rateprovided by the CCE. A base station determines a PDCCH format inaccordance with a DCI which is to be transmitted to a user equipment andattaches a CRC (cyclic redundancy check) to control information. The CRCis masked with an identifier named RNTI (radio network temporaryidentifier) in accordance with an owner or usage of the PDCCH. Forinstance, if the PDCCH is provided for a specific user equipment, theCRC may be masked with an identifier (e.g., cell-RNTI (C-RNTI)) of thecorresponding user equipment. In case that the PDCCH is provided for apaging message, the CRC may be masked with a paging indicator identifier(e.g., P-RNTI). If the PDCCH is provided for system information(particularly, for a system information block (SIC)), the CRC may bemasked with a system information identifier and a system informationRNTI (SI-RNTI). In order to indicate a random access response to atransmission of a random access preamble of a user equipment, the CRCmay be masked with RA-RNTI (random access-RNTI).

FIG. 4 is a diagram for a structure of an uplink (UL) subframe. A ULsubframe may be divided into a control region and a data region infrequency domain. A physical UL control channel (PUCCH) including ULcontrol information may be allocated to the control region. And, aphysical UL shared channel (PUSCH) including user data may be allocatedto the data region. In order to maintain single carrier property, oneuser equipment does not transmit PUCCH and PUSCH simultaneously. PUCCHfor one user equipment may be allocated to a resource block pair (RBpair) in subframe. Resource blocks belonging to the resource block pairmay occupy different subcarriers for 2 slots. Namely, a resource blockpair allocated to PUCCH is frequency-hopped on a slot boundary.

Reference Signal

In MIMO system, each transmitting antenna has an independent datachannel. A transmitting antenna may mean a virtual antenna or a physicalantenna. A receiver receives data transmitted from each transmittingantenna in a manner of estimating a channel for the correspondingtransmitting antenna. Channel estimation means a process forreconstructing a received signal by compensating for distortion of asignal caused by fading. In this case, the fading indicates an effectthat strength of a signal rapidly fluctuates due to multipath-time delayin a wireless communication system environment. For the channelestimation, a reference signal known to both a transmitter and areceiver is necessary. The reference signal may be simply named RS or apilot in accordance with an applicable standard.

In the legacy 3GPP LTE Release-8 or -9 system, a downlink referencesignal transmitted by a base station is defined. Downlink referencesignal is a pilot signal for coherent demodulation of such a channel asPDSCH (Physical Downlink Shared CHannel), PCFICH (Physical ControlFormat Indicator CHannel), PHICH (Physical Hybrid Indicator CHannel),PDCCH (Physical Downlink Control CHannel) and the like. The downlinkreference signal may be categorized into a common reference signal (CRS)shared by all user equipments in a cell and a dedicated reference signal(DRS) for a specific user equipment only. The common reference signalmay be called a cell-specific reference signal. And, the dedicatedreference signal may be called a user equipment-specific (UE-specific)reference signal or a demodulation reference signal (DMRS).

Downlink reference signal assignment in the legacy 3GPP LTE system isdescribed as follows. First of all, a position (i.e., a reference signalpattern) of a resource element for carrying a reference signal isdescribed with reference to one resource block pair (i.e., ‘one subframelength in time domain’×‘12-subcarrier length in frequency domain’). Asingle subframe is configured with 14 OFDM symbols (in case of a normalCP) or 12 OFDM symbols (in case of an extended CP). The number ofsubcarriers in a single OFDM symbol is set to one of 128, 256, 512,1024, 1536 and 2048 to use.

FIG. 5 shows a pattern of a common reference signal (CRS) in case that1-TTI (i.e., 1 subframe) has 14 OFDM symbols. FIG. 5 (a), FIG. 5 (b) andFIG. 5 (c) relates to a CRS pattern for a system having 1 Tx(transmitting) antenna, a CRS pattern for a system having 2 Tx antennasand a CRS pattern for a system having 4 Tx antennas, respectively.

In FIG. 5, R0 indicates a reference signal for an antenna port index 0.In FIG. 5, R1 indicates a reference signal for an antenna port index 1,R2 indicates a reference signal for an antenna port index 2, and R3indicates a reference signal for an antenna port index 3. Regarding aposition of an RE for carrying a reference signal for each of theantenna ports, no signal is transmitted from the rest of all antennaports except the antenna port for transmitting a reference signal toprevent interference.

FIG. 6 shows that a reference signal pattern is shifted in each cell toprevent reference signals of various cells from colliding with eachother. Assuming that a reference signal pattern for one antenna portshown in FIG. 5 (a) is used by a cell #1 (Cell 1) shown in FIG. 6, inorder to prevent collision of reference signals between cells includinga cell #2 adjacent to the cell #1, a cell #3 adjacent to the cell #1 andthe like, it is able to protect a reference signal in a manner ofshifting a reference signal pattern by subcarrier or OFDM symbol unit infrequency or time domain. For instance, in case of 1 Tx antennatransmission, since a reference signal is situated in 6-subcarrierinterval on a single OFDM symbol, if a shift by subcarrier unit infrequency domain is applied to each cell, at least 5 adjacent cells maybe able to situate reference signals on different resource elements,respectively. For instance, a frequency shift of a reference signal maybe represented as the cell #2 and the cell #6 in FIG. 6.

Moreover, by multiplying a downlink reference signal per cell by apseudo-random (PN) sequence and then transmitting the multiplied signal,interference caused to a receiver by a reference signal received from anadjacent cell can be reduced to enhance channel estimation performance.This PN sequence may be applicable by OFDM symbol unit in a singlesubframe. Regarding the PN sequence, a different sequence may beapplicable per cell ID, subframe number or OFDM symbol position.

In a system [e.g., 8-Tx antenna supportive wireless communication system(e.g., 3GPP LTE Release-10 system, a systems according to 3GPP LTEReleases next to Release-10, etc.)] having antenna configuration moreextended than a legacy 4-Tx antenna supportive communication system(e.g., 3GPP LTE Release-8 system, 3GPP LTE Release-9 system, etc.), DMRSbased data demodulation is taken into consideration to support efficientmanagement & operation and developed transmission scheme of referencesignals. In particular, in order to support data transmission viaextended antennas, it may be able to define DMRS for at least twolayers. Since DMRS is precoded by the same precoder of data, it is easyfor a receiving side to estimate channel information for demodulatingdata without separate precoding information. Meanwhile, a downlinkreceiving side is able to acquire channel information precoded for theextended antenna configuration through DMRS. Yet, a separate referencesignal other than the DMRS is requested to acquire non-precoded channelinformation. Hence, in a system by LTE-A standards, a reference signal(i.e., CSI-RS) for a receiving side to acquire channel state information(CSI) can be defined. In particular, CSI-RS may be transmitted via 8antenna ports. In order to discriminate a CSI-RS transmitted antennaport from an antenna port of 3GPP LTE Release-8/9, it may be able to useantenna port indexes 15 to 22.

Configuration of Downlink Control Channel

As a region for transmitting a downlink control channel, first threeOFDM symbols of each subframe are available. In particular, 1 to 3 OFDMsymbols are available in accordance with overhead of the downlinkcontrol channel. In order to adjust the number of OFDM symbols for adownlink control channel in each subframe, it may be able to use PCFICH.And, it is able to use PHICH to provide an acknowledgment response[ACK/NACK (acknowledgement/negative-acknowledgement)] to an uplinktransmission in downlink. Moreover, it is able to use PDCCH to transmitcontrol information for a downlink or uplink data transmission.

FIG. 7 and FIG. 8 show that the above-configured downlink controlchannels are assigned by resource element group (REG) unit in a controlregion of each subframe. FIG. 7 relates to a system having 1- or 2-Txantenna configuration and FIG. 8 relates to a system having 4-Tx antennaconfiguration. Referring to FIG. 7 and FIG. 8, REG corresponding to abasic resource unit for assigning a control channel is configured with 4contiguous Res in frequency domain except a resource element forassigning a reference signal. A specific number of REGs are availablefor a transmission of a downlink control channel in accordance withoverhead of the downlink control channel.

PCFICH (Physical Control Format Indicator Channel)

In order to provide every subframe with resource allocation informationof the corresponding subframe and the like, it is able to transmit PDCCHbetween OFDM symbol indexes 0 to 2. In accordance with overhead of acontrol channel, it may be able to use the OFDM symbol index 0, the OFDMsymbol indexes 0 and 1, or the OFDM symbol indexes 0 to 2. Thus, thenumber of OPFDM symbols used for a control channel is changeable foreach subframe. And, information on the OFDM symbol number may beprovided via PCFICH. Hence, the PCFICH should be transmitted in everysubframe.

Three kinds of informations can be provided through the PCFICH. Table 1in the following shows CFI (control format indicator) of PCFICH. ‘CFI=1’indicates that PDCCH is transmitted on OFDM symbol index 0, ‘CFI=2’indicates that PDCCH is transmitted on OFDM symbol indexes 0 and 1, and‘CFI=3’ indicates that PDCCH is transmitted on OFDM symbol indexes 0 to2.

[Table 1]

Information carried on PCFICH may be defined different in accordancewith a system bandwidth. For instance, in case that a bandwidth of asystem is smaller than a specific threshold, ‘CFI=1’ may indicate that 2OFDM symbols are used for PDCCH. ‘CFI =2’ may indicate that 3 OFDMsymbols are used for PDCCH. And, ‘CFI=3’ may indicate that 4 OFDMsymbols are used for PDCCH.

FIG. 9 is a diagram for a scheme of transmitting PCIFCH. REG shown inFIG. 9 is configured with 4 subcarriers, and more particularly, withdata subcarriers except RS (reference signal). Generally, a transmitdiversity scheme may apply thereto. A position of the REG may befrequency-shifted per cell (i.e., in accordance with a cell identifier)not to cause interference between cells. Additionally, PCFICH is alwaystransmitted on a 1^(st) OFDM symbol (i.e., OFDM symbol index 0) of asubframe. Hence, when a receiving end receives a subframe, the receivingend acquires the number of OFDM symbols for carrying PDCCH by checkinginformation of PCFICH and is then able to receive control informationtransmitted on the PDCCH.

PHICH (Physical Hybrid-ARO Indicator Channel)

FIG. 10 is a diagram to illustrate positions of PCFICH and PHICHgenerally applied for a specific bandwidth. ACK/NACK information on anuplink data transmission is transmitted on PHICH. Several PHICH groupsare created in a single subframe and several PHICHs exist in a singlePHICH group. Hence, PHICH channels for several user equipments areincluded in the single PHICH group.

Referring to FIG. 10, PHICH assignment for each user equipment inseveral PHICH groups are performed using a lowest PRB (physical resourceblock) index of PUSCH resource allocation and a cyclic shift index for ademodulation reference signal (DMRS) transmitted on a UL (uplink) grantPDCCH. In this case, the DMRS is a UL reference signal and is the signalprovided together with a UL transmission for channel estimation fordemodulation of UL data. Moreover, PHICH resource is known through suchan index pair as (n_(PHICH) ^(group), n_(PHICH) ^(seq)). In (n_(PHICH)^(group), n_(PHICH) ^(seq)), n_(PHICH) ^(group) means a PHICH groupnumber) and n_(PHICH) ^(seq) means an orthogonal sequence index in thecorresponding PHICH group. n_(PHICH) ^(group) and n_(PHICH) ^(seq) isdefined as Formula 1.n _(PHICH) ^(group)=(I _(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index) +n_(DMRS))mod N _(PHICH) ^(group)n _(PHICH) ^(seq)=(└I _(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index) /N_(PHICH) ^(group) ┘+n _(DMRS))mod 2N _(SF) ^(PHICH)

In Formula 1, n_(DMRS) indicates a cyclic shift of DMRS used for a PHICHassociated UL transmission. And, N_(SF) ^(PHICH) indicates a spreadingfactor size used for PHICH. I_(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index)indicates a lowest PRB index of a UL resource allocation. N_(PHICH)^(group) indicates the number of the configured PHICH groups and may bedefined as Formula 2.

$\begin{matrix}{N_{PHICH}^{group} = \left\{ \begin{matrix}\left\lceil {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right\rceil & {{for}\mspace{14mu}{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}} \\{2 \cdot \left\lceil {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right\rceil} & {{for}\mspace{14mu}{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \right.} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Formula 2, N_(g) indicates an amount of PHICH resource transmitted onPBCH (Physical Broadcast Channel) and N_(g) is represented as N_(g)ϵ{⅙,½, 1, 2} in 2-bit size.

One example of an orthogonal sequence defined by the legacy 3GPP LTERelease-8/9 is shown in Table 2.

TABLE 2 Orthogonal sequence Extended Sequence Normal cyclic index cyclicprefix prefix η_(PHICH) ^(seq) N_(SF) ^(PHICH) = 4 N_(SF) ^(PHICH) = 2 0[+1 +1 +1 +1] [+1 +1] 1 [+1 −1 +1 −1] [+1 −1] 2 [+1 +1 −1 −1] [+ j + j]3 [+1 −1 −1 +1] [+ j − j] 4 [+ j + j + j + j] — 5 [+ j − j + j − j] — 6[+ j + j − j − j] — 7 [+ j − j − j + j] —

FIG. 11 is a diagram to illustrate a position of a downlink (DL)resource element having PHICH group mapped thereto. Referring to FIG.11, PHICH group may be configured in different time region (i.e., adifferent OS (OFDM symbol)) within a single subframe.

PDCCH (Physical Downlink Control Channel)

Control information transmitted on PDCCH may have control informationsize and usage differing in accordance with a DCI (downlink controlinformation) format. And, a size of the PDCCH may vary in accordancewith a coding rate. For instance, DCI formats used by the legacy 3GPPLTE Release-8/9 may be defined as Table 3.

TABLE 3 DCI format Objectives 0 Scheduling of PUSCH 1 Scheduling of onePDSCH codeword 1A Compact scheduling of one PDSCH codeword 1BClosed-loop single-rank transmission 1C Paging, RACH response anddynamic BCCH 1D MU-MIMO 2 Scheduling of rank-adapted closed-loop spatialmultiplexing mode 2A Scheduling of rank-adapted open-loop spatialmultiplexing mode 3 TPC commands for PUCCH and PUSCH with 2 bit poweradjustments 3A TPC commands for PUCCH and PUSCH with single bit poweradjustments

The DCI format shown in Table 3 is independently applied per userequipment and PDCCHs of several user equipments can be simultaneouslymultiplexed within a single subframe. The multiplexed PDCCH of each ofthe user equipments is independently channel-coded and CRC is appliedthereto. The CRC of the PDCCH is masked with a unique identifier of eachof the user equipments and can be applied to enable the correspondinguser equipment to receive the PDCCH of its own. Yet, since a userequipment is basically unable to know a position of its PDCCH channel,the user equipment checks whether each of the entire PDCCH channels ofthe corresponding DCI format matches the PDCCH channel having the ID ofthe corresponding user equipment for each subframe and needs to performblind detection until receiving the corresponding PDCCH. A basicresource allocation unit of the PDCCH is CCE (control channel element)and a single CCE is configured with 9 TEGs. A single PDCCH may beconfigured with 1, 2, 4 or 8 CCEs. PDCCH configured in accordance witheach user equipment is interleaved into a control channel region of eachsubframe and then mapped by a CCE-to-RE mapping rule. This may vary anRE position having a CCE mapped thereto in accordance with the OFDMsymbol number for a control channel of each subframe, the PHICH groupnumber, Tx antennas, a frequency shift and the like.

Uplink Retransmission

Uplink (UL) retransmission may be indicated via the aforementioned PHICHand the DCI format 0 (i.e., DCI format for scheduling PUSCHtransmission). A user equipment receives ACKINACK for a previous ULtransmission via PHICH and is then able to perform a synchronousnon-adaptive retransmission. Alternatively, a user equipment receives aUL grant via DCI format 0 PDCCH from a base station and is then able toperform a synchronous adaptive retransmission.

The synchronous transmission means that a retransmission is performed ata predetermined timing point (e.g., (n+k)^(th) subframe) after a timingpoint (e.g., n^(th) subframe) of transmitting a data packet. In bothcases of the retransmission by PHICH and the retransmission by UL grantPDCCH, the synchronous retransmission is performed.

Regarding the non-adaptive retransmission of performing a retransmissionon PHICH, the same frequency resource and transmitting method of theformer frequency resource (e.g., physical resource block (PRB) region)and transmitting method (e.g., modulation scheme, etc.) used for aprevious transmission are applied to the retransmission. Meanwhile,regarding the adaptive retransmission of performing a retransmission viaUL grant PDCCH, a frequency resource and transmitting method forperforming a retransmission may be set different from those of aprevious transmission in accordance with a scheduling controlinformation indicated by a UL grant.

In case that a user equipment receives a UL grant PDCCH as soon asreceives PHICH, the user equipment may be able to perform a ULtransmission in accordance with control information of the UL grantPDCCH by ignoring the PHICH. Since a new data indicator (NDI) isincluded in the UL grant PDCCH (e.g., DCI format 0), if NDI bit istoggled more than a previously provided NDI value, the user equipmentregards a previous transmission as successful and is then able totransmit new data. Meanwhile, although the user equipment receives ACKfor a previous transmission via PHICH, unless the NDI value is toggledin the UL grant PDCCH received simultaneously with or after the PHICHreception, the user equipment is configured not to flush a buffer forthe previous transmission.

Uplink Transmission Configuration

FIG. 12 is a diagram for a structure of a transmitter by SC-FDMA.

First of all, a single block configured with N symbols inputted to atransmitter is converted to a parallel signal via a serial-to-parallelconverter 1201. The parallel signal spreads via an N-point DFT module1202. The spreading signal is mapped to a frequency region via asubcarrier mapping module 1203. Signals on subcarriers configure linearcombination of N symbols. The signal mapped to the frequency region istransformed into a time-domain signal via an M-point IFFT module 1204.The time-domain signal is converted to a parallel signal via aparallel-to-serial converter 1205 and then has a CP added thereto. Theeffect of the IFFT processing by the M-point IFFT module 1204 iscancelled out by the DFT processing of the N-point DFT module 1202 tosome extent. In this point, the SC-FDMA may be named DFT-s-OFDMA(DFT-spread-OFDMA). Moreover, although the signal inputted to the DFTmodule 1202 has a low PAPR (peak-to-average power ratio) or CM (cubicmetric), it may have a high PAPR after DFT processing. And, the signaloutputted by the IFFT processing of the IFFT module 1204 may have a lowPAPR again. In particular, according to the SC-FDMA, transmission isperformed by avoiding a non-linear distortion interval of a poweramplifier (PA), whereby a cost for implementation of a transmitting endcan be reduced.

FIG. 13 is a diagram to describe a scheme of mapping a signal outputtedfrom the DFT module 1202 to a frequency region. By performing one of thetwo schemes shown in FIG. 13, a signal outputted from an SC-FDMAtransmitter can meet the single carrier property. FIG. 13 (a) shows alocalized mapping scheme of locally mapping signals outputted from theDFT module 1202 to a specific part of a subcarrier region. FIG. 13 (b)shows a distributed mapping scheme of mapping signals outputted from theDFT module 1202 to a whole subcarrier region by being distributed. Inthe legacy 3GPP LTE Release-8/9 system, it is defined that the localizedmapping scheme is used.

FIG. 14 is a block diagram to describe a transmission processing of areference signal to demodulate a transmitted signal by SC-FDMA. In thelegacy 3GPP LTE Release-8/9 system, a data part is transmitted in amanner of transforming a signal generated from a time domain into afrequency-domain signal by DFT processing, performing subcarrier mappingon the frequency-domain signal, and then performing IFFT processing onthe mapped signal [cf. FIG. 12]. Yet, a reference signal (RS) is definedas directly generated in frequency domain without DFT processing, mappedto subcarrier, undergoing IFFT processing, and then having CP additionthereto.

FIG. 15 is a diagram to illustrate a symbol position having a referencesignal (RS) mapped thereto in a subframe structure according to SC-FDMA.FIG. 15 (a) shows that RS is located at 4^(th) SC-FDMA symbol of each of2 slots in a single subframe in case of a normal CP. FIG. 15 (b) showsthat RS is located at 3^(rd) SC-FDMA symbol of each of 2 slots in asingle subframe in case of an extended CP.

Clustered DFT-s-OFDMA scheme is described with reference to FIGS. 16 to19 as follows. The clustered DFT-s-OFDMA is the modification of theaforementioned SC-FDMA. According to the clustered DFT-s-OFDMA, aDFT-processed signal is segmented into a plurality of sub-blocks andthen mapped to a spaced position in frequency domain.

FIG. 16 is a diagram to describe a clustered DFT-s-OFDMA scheme on asingle carrier. For instance, a DFT output may be partitioned into Nsbsub-blocks (sub-blocks #0 to #Nsb-1). When sub-blocks are mapped to afrequency region, the sub-blocks #0 to #NSb-1 are mapped to a singlecarrier (e.g., carrier of 20 MHz bandwidth, etc.) and each of thesub-blocks may be mapped to a position spaced in the frequency region.And, each of the sub-blocks may be locally mapped to the frequencyregion.

FIG. 17 and FIG. 18 are diagrams to describe a clustered DFT-s-OFDMAscheme on multiple carriers.

In a situation (i.e., frequency bands of multiple carriers (cells) arecontiguously assigned) that multiple carriers (or multiple cells) areconfigured contiguously, if a subcarrier interval between contiguouscarriers is aligned, FIG. 17 is a diagram for one example that a signalcan be generated through a single IFFT module. For instance, a DFToutput may be segmented into Nsb sub-blocks (sub-blocks #0 to #NSb-1).In mapping sub-blocks to a frequency region, the sub-blocks #0 to #NSb-1can be mapped to component carriers #0 to #NSb-1, respectively [e.g.,each carrier (or cell) may have a bandwidth of 20 MHz]. moreover, eachof the sub-blocks may be mapped to a frequency region by beinglocalized. And, the sub-blocks mapped to the carriers (or cells) may betransformed into a time-domain signal through a single IFFT module.

In a situation (i.e., frequency bands of multiple carriers (cells) arenon-contiguously assigned) that multiple carriers (or multiple cells)are configured non-contiguously, FIG. 18 is a diagram for one examplethat a signal is generated using a plurality of IFFT modules. Forinstance, a DFT output may be segmented into Nsb sub-blocks (sub-blocks#0 to #NSb-1). In mapping sub-blocks to a frequency region, thesub-blocks #0 to #NSb-1 can be mapped to carriers #0 to #NSb-1,respectively [e.g., each carrier (or cell) may have a bandwidth of 20MHz]. moreover, each of the sub-blocks may be mapped to a frequencyregion by being localized. And, the sub-blocks mapped to the carriers(or cells) may be transformed into a time-domain signal through the IFFTmodules, respectively.

If the clustered DFT-s-OFDMA on the single carrier mentioned withreference to FIG. 16 is called intra-carrier (or intra-cell)DFT-s-OFDMA, the DFT-s-OFDMA on the multiple carriers (or cells)mentioned with reference to FIG. 17 or FIG. 18 may be calledinter-carrier (or inter-cell) DFT-s-OFDMA. Thus, the intra-carrierDFT-s-OFDMA and the inter-carrier DFT-s-OFDMA may be interchangeablyusable.

FIG. 19 is a diagram to describe a chuck-specific DFT-s-OFDMA scheme ofperforming DFT processing, frequency domain mapping and IFFT processingby chunk unit. The chunk-specific DFT-s-OFDMA may be called Nx SC-FDMA.A code block segmented signal is chunk-segmented into parts and channelcoding and modulation is performed on each of the parts. The modulatedsignal undergoes the DFT processing, the frequency domain mapping andthe IFFT processing in the same manner described with reference to FIG.12, outputs from the respective IFFTs are added up, and CP may be addedthereto. The Nx SC-FDMA scheme mentioned with reference to FIG. 19 maybe applicable to a contiguous multi-carrier (or multi-cell) case and anon-contiguous multi-carrier (or multi-cell) case both.

Structure of MIMO System

FIG. 20 is a diagram to illustrate a basic structure of MIMO systemhaving multiple Tx antennas and/or multiple Rx (receiving) antennas.Each block shown in FIG. 20 conceptionally indicates a function oroperation in a transmitting/receiving end for MIMO transmission.

A channel encoder shown in FIG. 20 indicates an operation of attaching aredundancy bit to an input data bit, whereby effect of noise and thelike from a channel can be reduced. A mapper indicates an operation ofconverting data bit information to data symbol information. Aserial-to-parallel converter indicates an operation of converting serialdata to parallel data. A multi-antenna encoder indicates an operation oftransforming a data symbol into a time-spatial signal. A multi-antennaof a transmitting end plays a role in transmitting this time-spatialsignal on a channel, while a multi-antenna of a receiving end plays arole in receiving the signal on the channel.

A multi-antenna decoder shown in FIG. 20 indicates an operation oftransforming the received time-spatial signal into each data symbol. Aparallel-to-serial converter indicates an operation of converting aparallel signal to a serial signal. A demapper indicates an operation oftransforming a data symbol to a bit information. A decoding operationfor a channel code is performed by a channel decoder, whereby data canbe estimated.

The MIMO transceiving system mentioned in the above description may havea single or several codewords spatially in accordance with a spacemultiplexing ratio. A case of having a single codeword spatially iscalled a single codeword (SCW) structure. And, a case of having severalcodewords is called a multiple codeword (MCW) structure.

FIG. 21 (a) is a block diagram to represent functionality of atransmitting end of an MIMO system having the SCW structure. And, FIG.21 (b) is a block diagram to represent functionality of a transmittingend of an MIMO system having the MCW structure.

Codebook Based Precoding Scheme

In order to support multi-antenna transmission, it may be able to applyprecoding of appropriately distributing transmission information to eachantenna in accordance with a channel status and the like. A codebookbased precoding scheme means the scheme performed in a following manner.First of all, a set of precoding matrixes is determined in atransmitting end and a receiving end. Secondly, the transmitting endmeasures channel information from the transmitting end and then feedsback information (i.e., a precoding matrix index (PMI)) indicating whatis a most appropriate precoding matrix to the transmitting end. Finally,the transmitting end applies an appropriate precoding to a signaltransmission based on the PMI. Since the appropriate precoding matrix isselected from the previously determined precoding matrix set, althoughan optimal precoding is not always applied, this is more advantageousthan the explicit feedback of optimal precoding information actuallycarried on channel information in reducing feedback overhead.

FIG. 22 is a diagram to describe basic concept of codebook basedprecoding.

According to a codebook based precoding scheme, a transmitting and areceiving end share codebook information including a prescribed numberof precoding matrixes in accordance with a transmission rank, the numberof antennas and the like. In particular, in case that feedbackinformation is finite, it is able to use the precoding based codebookscheme. The receiving end measures a channel status via a receivedsignal and is then able to deed back information (i.e., indexes of thecorresponding precoding matrixes) on the finite number of preferredprecoding matrixes based on the above-mentioned codebook information tothe transmitting end. For instance, the receiving end is able to selectan optimal precoding matrix in a manner of measuring a received signalby ML (maximum likelihood) or MMSE (minimum mean square error) scheme.FIG. 22 shows that the receiving end transmits the precoding matrixinformation per codeword to the transmitting end, by which the presentinvention may be non-limited.

Having received the feedback information from the receiving end, thetransmitting end may be able to select a specific precoding matrix fromthe codebook based on the received information. Having selected theprecoding matrix, the transmitting end performs a precoding in a mannerof multiplying layer signals, of which number corresponds to thetransmission rank, by the selected precoding matrix and may be then ableto transmit a precoded transmission signal via a plurality of antennas.In the precoding matrix, the number of rows is equal to that of theantennas and the number of columns is equal to a rank value. Since therank value is equal to the number of the layers, the number of thecolumns is equal to the number of layers. For instance, if the number ofthe Tx antennas and the number of the transmission layers are 4 and 2,respectively, the precoding matrix can be configured with 4×2 matrix.Information transmitted via each layer can be mapped to each antenna viathe precoding matrix.

Having received the signal precoded and transmitted by the transmittingend, the receiving end is able to reconstruct the received signal byperforming a processing inverse to that of the precoding performed bythe transmitting end. Generally, since the precoding matrix meets such aunitary matrix (U) condition as U*U^(H)=I, the inverse processing of theprecoding may be performed in a manner of multiplying the receivedsignal by Hermit matrix (P^(H)) of the precoding matrix (P) used for theprecoding of the transmitting end.

For instance, Table 4 in the following indicates a codebook used for adownlink transmission using 2 Tx antennas in 3GPP LTE Release-8/9 andTable 5 indicates a codebook used for a downlink transmission using 4 Txantennas in 3GPP LTE Release-8/9.

TABLE 4 Number of rank Codebook index 1 2 0$\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}$ $\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$ 1 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ 2 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}$ 3 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}$ —

TABLE 5 Codebook Number of layers υ index u_(n) 1 2 3 4  0$u_{0} = \begin{bmatrix}1 & {- 1} & {- 1} & {- 1}\end{bmatrix}^{T}$ W₀ ^({1}) W₀ ^({14})/{square root over (2)} W₀^({124})/{square root over (3)} W₀ ^({1234})/2  1$u_{1} = \begin{bmatrix}1 & {- j} & 1 & j\end{bmatrix}^{T}$ W₁ ^({1}) W₁ ^({12})/{square root over (2)} W₁^({123})/{square root over (3)} W₁ ^({1234})/2  2$u_{2} = \begin{bmatrix}1 & 1 & {- 1} & 1\end{bmatrix}^{T}$ W₂ ^({1}) W₂ ^({12})/{square root over (2)} W₂^({123})/{square root over (3)} W₂ ^({3214})/2  3$u_{3} = \begin{bmatrix}1 & j & 1 & {- j}\end{bmatrix}^{T}$ W₃ ^({1}) W₃ ^({12})/{square root over (2)} W₃^({123})/{square root over (3)} W₃ ^({3214})/2  4$u_{4} = \begin{bmatrix}1 & {\left( {{- 1} - j} \right)/\sqrt{2}} & {- j} & {\left( {1 - j} \right)/\sqrt{2}}\end{bmatrix}^{T}$ W₄ ^({1}) W₄ ^({14})/{square root over (2)} W₄^({124})/{square root over (3)} W₄ ^({1234})/2  5$u_{5} = \begin{bmatrix}1 & {\left( {1 - j} \right)/\sqrt{2}} & j & {\left( {{- 1} - j} \right)/\sqrt{2}}\end{bmatrix}^{T}$ W₅ ^({1}) W₅ ^({14})/{square root over (2)} W₅^({124})/{square root over (3)} W₅ ^({1234})/2  6$u_{6} = \begin{bmatrix}1 & {\left( {1 + j} \right)/\sqrt{2}} & {- j} & {\left( {{- 1} + j} \right)/\sqrt{2}}\end{bmatrix}^{T}$ W₆ ^({1}) W₆ ^({13})/{square root over (2)} W₆^({134})/{square root over (3)} W₆ ^({1324})/2  7$u_{7} = \begin{bmatrix}1 & {\left( {{- 1} + j} \right)/\sqrt{2}} & j & {\left( {1 + j} \right)/\sqrt{2}}\end{bmatrix}^{T}$ W₇ ^({1}) W₇ ^({13})/{square root over (2)} W₇^({134})/{square root over (3)} W₇ ^({1324})/2  8$u_{8} = \begin{bmatrix}1 & {- 1} & 1 & 1\end{bmatrix}^{T}$ W₈ ^({1}) W₈ ^({12})/{square root over (2)} W₈^({124})/{square root over (3)} W₈ ^({1234})/2  9$u_{9} = \begin{bmatrix}1 & {- j} & {- 1} & {- j}\end{bmatrix}^{T}$ W₉ ^({1}) W₉ ^({14})/{square root over (2)} W₉^({134})/{square root over (3)} W₉ ^({1234})/2 10$u_{10} = \begin{bmatrix}1 & 1 & 1 & {- 1}\end{bmatrix}^{T}$ W₁₀ ^({1}) W₁₀ ^({13})/{square root over (2)} W₁₀^({123})/{square root over (3)} W₁₀ ^({1324})/2 11$u_{11} = \begin{bmatrix}1 & j & {- 1} & j\end{bmatrix}^{T}$ W₁₁ ^({1}) W₁₁ ^({13})/{square root over (2)} W₁₁^({134})/{square root over (3)} W₁₁ ^({1324})/2 12$u_{12} = \begin{bmatrix}1 & {- 1} & {- 1} & 1\end{bmatrix}^{T}$ W₁₂ ^({1}) W₁₂ ^({12})/{square root over (2)} W₁₂^({123})/{square root over (3)} W₁₂ ^({1234})/2 13$u_{13} = \begin{bmatrix}1 & {- 1} & 1 & {- 1}\end{bmatrix}^{T}$ W₁₃ ^({1}) W₁₃ ^({13})/{square root over (2)} W₁₃^({123})/{square root over (3)} W₁₃ ^({1324})/2 14$u_{14} = \begin{bmatrix}1 & 1 & {- 1} & {- 1}\end{bmatrix}^{T}$ W₁₄ ^({1}) W₁₄ ^({13})/{square root over (2)} W₁₄^({123})/{square root over (3)} W₁₄ ^({3214})/2 15$u_{15} = \begin{bmatrix}1 & 1 & 1 & 1\end{bmatrix}^{T}$ W₁₅ ^({1}) W₁₅ ^({12})/{square root over (2)} W₁₅^({123})/{square root over (3)} W₁₅ ^({1234})/2

In Table 5, W_(n) ^({s}) is obtained from a set {S} configured from aformula expressed as W_(n)=I−2u_(n)u_(n) ^(H)/u_(n) ^(H)u_(n). In thiscase, I indicates 4×4 unitary matrix and u_(n) indicates a value givenby Table 5.

Referring to Table 4, when a codebook for 2 Tx antennas has total 7precoding vectors/matrixes, since a unitary matrix is provided for anopen-loop system, there are total 6 precoding vectors/matrixes for theprecoding of a closed-loop system. Moreover, referring to Table 5, acodebook for 4 Tx antennas has total 64 precoding vectors/matrixes.

The above-mentioned codebooks have such a common property as a constantmodulus (CM) property, a nested property, a constrained alphabetproperty and the like. According to the CM property, each element ofevery precoding matrix within a codebook does not contain ‘0’ and isconfigured to have the same size. According to the nested property, aprecoding matrix of a low rank is designed to be configured with asubset of a specific column of a precoding matrix of a high rank.According to the constrained alphabet property, each element of everyprecoding matrix within a codebook is constrained. For instance, eachelement of a precoding matrix is limited only to an element (±1) usedfor BPSH (binary phase shift keying), elements (±1, ±j) used for QPSK(quadrature phase shift keying), or elements

$\left( {{\pm 1},{\pm j},{\pm \frac{\left( {1 + j} \right)}{\sqrt{2}}},{\pm \frac{\left( {{- 1} + j} \right)}{\sqrt{2}}}} \right)$used for 8-PSK. In the example of the codebook shown in Table 5, sincealphabet of each element of every precoding matrix within the codebookis configured with

$\left\{ {{\pm 1},{\pm j},{\pm \frac{\left( {1 + j} \right)}{\sqrt{2}}},{\pm \frac{\left( {{- 1} + j} \right)}{\sqrt{2}}}} \right\},$it may be represented as having the constrained alphabet property.

Feedback Channel Structure

Basically, since a base station is unable to know information on a DLchannel in FDD system, channel information fed back by a user equipmentis used for a DL transmission. In case of the legacy 3GPP LTERelease-8/9 system, it is able to feed back DL channel information viaPUCCH or PUSCH. In case of the PUCCH, channel information isperiodically fed back. In case of the PUSCH, channel information isaperiodically fed back in accordance with a request made by a basestation. Moreover, feedback of channel information may be performed in amanner of feeding back the channel information on a whole frequency band(i.e., wideband (WB)) or the channel information on a specific number ofRBs (i.e., subband (SB)).

Extended Antenna Configuration

FIG. 23 shows examples of configuration of 8 Tx (transmitting) antennas.

FIG. 23 (a) shows a case that N antennas configure independent channelswithout being grouped, which is generally called ULA (uniform lineararray). In case that the number of antennas is small, the ULAconfiguration may be available. In case that the number of antennas isbig, multiple antennas are arranged in a manner being spaced apart fromeach other. Hence, it may be insufficient for a space of a transmitterand/or receiver to configure independent channels.

FIG. 23 (b) shows an antenna configuration (i.e., paired ULA) of ULAtype in which 2 antennas form a pair. In this case, an associatedchannel is established between a pair of the antennas and may beindependent from that of antennas of another pair.

Meanwhile, unlike the fact that the legacy 3GPP LTE Release-8/9 systemuses 4 Tx antennas in DL, the 3GPP LTE Release-10 system is able to use8 Tx antennas in DL. In order to apply this extended antennaconfiguration, it is necessary to install several Tx antennas ininsufficient space. Thus, the ULA antenna configuration shown FIG. 23(a) or FIG. 23 (b) may become inappropriate. Therefore, it may be ableto consider applying a dual-polarized (or cross-polarized) antennaconfiguration shown in FIG. 23 (c). In case of this configuration of Txantennas, even if a distance between antennas is relatively short, it isable to configure independent channels by lowering correlation.Therefore, data transmission of high throughput can be achieved.

Codebook Structures

If a transmitting end and a receiving end share a pre-defined codebookwith each other, it is able to lower an overhead for the receiving endto feed back precoding information to be used for MIMO transmission fromthe transmitting end. Hence, it is able to apply efficient precoding.

For one example of configuring a pre-defined codebook, it is able toconfigure a precoder matrix using DFT (discrete Fourier transform)matrix or Walsh matrix. Alternatively, it is able to configure precodersof various types by combination with a phase shift matrix, a shiftdiversity matrix or the like.

In configuring a DFT matrix based codebook, n×n DFT matrix can bedefined as Formula 3.

$\begin{matrix}{{{{DFTn}\text{:}\mspace{14mu}{D_{n}\left( {k,\ell} \right)}} = {\frac{1}{\sqrt{n}}{\exp\left( {{- j}\; 2\pi\; k\;{\ell/n}} \right)}}},k,{\ell = 0},1,\ldots\mspace{11mu},{n - 1}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In the DFT matrix of Formula 3, a single matrix exists for a specificsize n only. Hence, in order to appropriately define and use variouskinds of precoding matrixes in accordance with a situation, it may beable to consider configuring to use a rotated version of the DFTn matrixin addition. One example of the rotated DFTn matrix is shown in Formula4.

$\begin{matrix}{{{{rotated}\mspace{14mu}{DFTn}\text{:}\mspace{14mu}{D_{n}^{({G,g})}\left( {k,\ell} \right)}} = {\frac{1}{\sqrt{n}}{\exp\left( {{- j}\; 2\pi\;{{k\left( {\ell + {g/G}} \right)}/n}} \right)}}},\mspace{20mu} k,{\ell = 0},1,\ldots\mspace{11mu},{n - 1},{g = 0},1,\ldots\mspace{11mu},{G.}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In case that the DFT matrix shown in Formula 4 is configured, it is ableto create G rotated DFTn matrixes. And, the created matrixes may meetthe property of the DFT matrix.

In the following description, a householder-based codebook structure isexplained. The householder-based codebook structure may mean thecodebook configured with householder matrix. In particular, thehouseholder matrix is the matrix used for householder transform. Thehouseholder transform is a sort of linear transformation and may beusable in performing QR decomposition. The QR decomposition may meanthat a prescribed matrix is decomposed into an orthogonal matrix (Q) andan upper triangular matrix (R). The upper triangular matrix means asquare matrix of which components below main diagonal components are allzeros. One example of the 4×4 householder matrix is shown in Formula 5.

$\begin{matrix}{{M_{1} = {{I_{4} - {2\; u_{0}{u_{1}^{H}/{u_{0}}^{2}}}} = {\frac{1}{\sqrt{4}}*\begin{bmatrix}1 & 1 & 1 & 1 \\1 & 1 & {- 1} & {- 1} \\1 & {- 1} & 1 & {- 1} \\1 & {- 1} & {- 1} & 1\end{bmatrix}}}},\mspace{20mu}{u_{0}^{T} = \begin{bmatrix}1 & {- 1} & {- 1} & {- 1}\end{bmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

It is able to create 4×4 unitary matrix having the CM property byhouseholder transformation. Like the codebook for the 4 Tx antennasshown in Table 5, n×n precoding matrix is created using the householdertransformation and a column subset of the created precoding matrix canbe configured to be used as a precoding matrix for a transmission of arank smaller than n.

Multi-Codebook Based Precoder Creation

A precoding operation used for a multi-antenna transmission may beexplained as an operation of mapping a signal transmitted via layer(s)to antenna(s). In particular, by X×Y precoding matrix, Y transmissionlayers (or streams) can be mapped to X Tx antennas.

In order to configure N_(t)×R precoding matrix used in transmitting Rstreams (i.e., Rank R) via N_(t) Tx antennas, a transmitting endreceives a feedback of at least one precoding matrix index (PMI) from areceiving end and is then able to configure a precoder matrix. Formula 6shows one example of a codebook configured with n_(c) matrixes.P _(N) _(t) _(×R)(k)ϵ{P ₁ ^(N) ^(t) ^(×R) ,P ₂ ^(N) ^(t) ^(×R) ,P ₃ ^(N)^(t) ^(×R) , . . . ,P _(n) _(c) ^(N) ^(t) ^(×R)}  [Formula 6]

In Formula 6, k indicates a specific resource index (e.g., a subcarrierindex, a virtual resource index, a subband index, etc.). Formula 6 maybe configured in form of Formula 7.

$\begin{matrix}{{{P_{N_{t} \times R}(k)} = \begin{bmatrix}P_{{M_{t} \times R},1} \\P_{{M_{t} \times R},2}\end{bmatrix}},{N_{t} = {2 \cdot M_{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack\end{matrix}$

In Formula 7, P_(M) _(t) _(×R,2) may be configured in form of shiftingP_(M) _(t) _(×R,1) by a specific complex weight w₂. Hence, if adifference between P_(M) _(t) _(×R,1) and P_(M) _(t) _(×R,2) isrepresented as a specific complex weight, it may be expressed as Formula8.

$\begin{matrix}{{P_{N_{t} \times R}(k)} = \begin{bmatrix}{w_{1} \cdot P_{{M_{t} \times R},1}} \\{w_{2} \cdot P_{{M_{t} \times R},1}}\end{bmatrix}} & \left\lbrack {{formula}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Moreover, Formula 8 may be represented as Formula 9 using Kronekerproduct (⊗).

$\begin{matrix}{{P_{{N_{t} \times R},n,m}(k)} = {{\begin{bmatrix}w_{1} \\w_{2}\end{bmatrix} \otimes P_{{M_{t} \times R},1}} = {W_{n} \otimes P_{m}}}} & \left\lbrack {{Formula}\mspace{14mu} 9} \right\rbrack\end{matrix}$

Kroneker product is an operation for 2 matrixes in random size. As aresult of Kroneker product operation, it is able to obtain a blockmatrix. For instance, Kroneker product of an m×n matrix A and a p×qmatrix B (i.e., A⊗B) may be represented as Formula 10. In Formula 10,a_(mn) indicates an element of the matrix A and b_(pq) indicates anelement of the matrix B.

$\begin{matrix}{{A \otimes B} = \left\lbrack \begin{matrix}{a_{11}b_{11}} & {a_{11}b_{12}} & \ldots & {a_{11}b_{1q}} & \ldots & \ldots & {a_{1n}b_{11}} & {a_{1n}b_{12}} & \ldots & {a_{1n}b_{1q}} \\{a_{11}b_{21}} & {a_{11}b_{22}} & \ldots & {a_{11}b_{2q}} & \ldots & \ldots & {a_{1n}b_{21}} & {a_{1n}b_{22}} & \ldots & {a_{1n}b_{2q}} \\\vdots & \vdots & \ddots & \vdots & \; & \; & \vdots & \vdots & \ddots & \vdots \\{a_{11}b_{p\; 1}} & {a_{11}b_{p\; 2}} & \ldots & {a_{11}b_{pq}} & \ldots & \ldots & {a_{1n}b_{p\; 1}} & {a_{1n}b_{p\; 2}} & \ldots & {a_{1n}b_{pq}} \\\vdots & \vdots & \; & \vdots & \ddots & \; & \vdots & \vdots & \; & \vdots \\\vdots & \vdots & \; & \vdots & \; & \ddots & \vdots & \vdots & \; & \vdots \\{a_{m\; 1}b_{11}} & {a_{m\; 1}b_{12}} & \ldots & {a_{m\; 1}b_{1q}} & \ldots & \ldots & {a_{mn}b_{11}} & {a_{mn}b_{12}} & \ldots & {a_{mn}b_{1q}} \\{a_{m\; 1}b_{21}} & {a_{m\; 1}b_{22}} & \ldots & {a_{m\; 1}b_{2q}} & \ldots & \ldots & {a_{mn}b_{21}} & {a_{mn}b_{22}} & \ldots & {a_{mn}b_{2q}} \\\vdots & \vdots & \ddots & \vdots & \; & \; & \vdots & \vdots & \ddots & \vdots \\{a_{m\; 1}b_{p\; 1}} & {a_{m\; 1}b_{p\; 2}} & \ldots & {a_{m\; 1}b_{pq}} & \ldots & \ldots & {a_{mn}b_{p\; 1}} & {a_{mn}b_{p\; 2}} & \ldots & {a_{mn}b_{pq}}\end{matrix} \right\rbrack} & \left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack\end{matrix}$

A partial matrix

$\quad\begin{bmatrix}w_{1} \\w_{2}\end{bmatrix}$of precoding and P_(M) _(t) _(×R,1) in Formula 9 may be independentlyfed back from a receiving end. And, a transmitting end is able toconfigure and use a precoder like Formula 8 or Formula 9 using eachfeedback information. In case of applying the form of Formula 8 orFormula 9, W is always configured in form of 2×1 vector and may beconfigured in form of a codebook shown I Formula 11.

$\begin{matrix}{{W \in \begin{bmatrix}1 \\e^{j\frac{2\pi}{N}i}\end{bmatrix}},{i = 0},\ldots\mspace{14mu},{N - 1}} & \left\lbrack {{Formula}\mspace{14mu} 11} \right\rbrack\end{matrix}$

In Formula 11, N indicates the total number of precoding vectorscontained in the codebook and i may be used as an index of a vector. Inorder to obtain proper performance by minimizing feedback overhead, imay be usable by being set to 2, 4 or 8. Moreover, P_(M) _(t) _(×R,1)may be configured as a codebook for 4 Tx antennas or a codebook for 2 Txantennas. For this, the codebook of Table 4 or Table 5 (e.g., thecodebook for 2 or 4 Tx antennas defined in 3GPP LTE Release-8/9) isusable. And, P_(M) _(t) _(×R,1) may be configured in rotated DFT form aswell.

Moreover, a matrix W may be available in form of 2×2 matrix. Formula 12shows one example of the 2×2 matrix W.

$\begin{matrix}{{{P_{{N_{t} \times 2R},n,m}(k)} = {{\begin{bmatrix}w_{1} & w_{3} \\w_{2} & w_{4}\end{bmatrix} \otimes P_{{M_{t} \times R},1}} = {W_{n} \otimes P_{m}}}},{N_{t} = {2 \cdot M_{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 12} \right\rbrack\end{matrix}$

In case of the configuration of Formula 12, if a maximum rank of thecodebook P_(M) _(t) _(×R,1) is R, it may be able to design a codebook ofa rank up to 2R. For instance, in case of using the codebook shown inTable 4 as P_(M) _(t) _(×R,1), according to Formula 9, it may be usableup to 4 (R=4) as a maximum rank. On the other hand, according to Formula12, it may be able to use a maximum rank up to 8 (2R=8). Hence, in thesystem configured with 8 Tx antennas, it is able to configure a precodercapable of 8×8 transmission. In this case, W may be configured in formof a codebook shown in Formula 13.

$\begin{matrix}{{W \in \begin{bmatrix}1 & 1 \\e^{j\frac{2\pi}{N}i} & {- e^{j\frac{2\pi}{N}i}}\end{bmatrix}},{i = 0},\ldots\mspace{14mu},{N - 1}} & \left\lbrack {{Formula}\mspace{14mu} 13} \right\rbrack\end{matrix}$

The precoder configuring method according to Formula 9 or Formula 12 mayapply differently in accordance with each rank. For instance, the methodof Formula 9 is used for a case of a rank equal to or lower than 4(R≤4). And, the method of Formula 12 may be used for a case of a rankequal to or higher than 5 (R≥5). Alternatively, the method of Formula 9is used only for a case of a rank 1 (R=1). In other cases (i.e., rank 2or higher (R≥2)), it may be able to use the method of Formula 12. The Wand P mentioned in association with Formula 9 and Formula 12 may be fedback to have the property as shown in Table 6.

TABLE 6 Case W/P Frequency One of two matrixes may be configured to befed granularity 1 back on subband and the other may be configured to befed back on wideband. Frequency One of two matrixes may be configured tobe fed granularity 2 back on nest-M band and the other may be configuredto be fed back on wideband. Time One of two matrixes may be configuredto be fed granularity back by periods N and the other may be configuredto be fed back by periods M. Feedback One of two matrixes may beconfigured to be fed back on channel 1 PUSCH and the other may beconfigured to be fed back on PUCCH. Feedback In case of feedback onPUSCH, one (e.g., W) of two channel 2 matrixes may be configured to befed back on subband and the other (e.g., P) may be configured to be fedback on wideband. In case of feedback on PUCCH, both Q and P may beconfigured to be fed back on wideband. Unequal One (e.g., P) of twomatrixes may be configured to be coded protection at a more reliablerating rate and the other (e.g., W) may be configured to be coded at arelatively low coding rate. Alphabet Alphabet of a matrix W may beconfigured to be restriction 1 constrained by BPSK and alphabet of amatrix P may be configured to be constrained by QPSK or 8 PSK. AlphabetAlphabet of a matrix W may be configured to be restriction 2 constrainedby QPSK and alphabet of a matrix P may be configured to be constrainedby QPSK or 8 PSK.

In the following description, a multi-codebook based precoder having thenested property is explained.

First of all, it is able to configure a codebook using the method ofFormula 9 and the method of Formula 12 appropriately. Yet, in somecases, it may be impossible to configure a precoder unless using twokinds of combinations. To solve this problem, it may be able toconfigure and use a precoder as shown in Formula 14.

$\begin{matrix}{{P_{{N_{t} \times N_{t}},n,m} = {{\begin{bmatrix}w_{1} & w_{3} \\w_{2} & w_{4}\end{bmatrix} \otimes P_{M_{t} \times M_{t}}} = {W_{n} \otimes P_{m}}}},{N_{t} = {2 \cdot M_{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 14} \right\rbrack\end{matrix}$

A precoder for a case that a rank value is equal to the number of Txantennas (R=N_(t)) is configured using the P_(N) _(t) _(×N) _(t)obtained from Formula 14 and a column subset of the configured precodermay be usable as a precoder for a lower rank. If the precoder isconfigured in the above manner, the nested property can be met tosimplify the CQI calculation. In Formula 14, P_(N) _(t) _(×N) _(t)_(,n,m) indicates the precoder in case of R=N_(t). In this case, forexample, a subset configured with 0^(th) and 2^(nd) columns of P_(N)_(t) _(×N) _(t) _(,n,m) may be usable for a precoder for R=2, which canbe represented as P_(N) _(t) _(×N) _(t) _(,n,m)(0,2). In this case,P_(M) _(t) _(×M) _(t) may be configured with a rotated DFT matrix or acodebook of another type.

Meanwhile, in order to raise a diversity gain in an open-loopenvironment, based on the precoder configured in the above manner, it isable to maximize the beam diversity gain by exchanging to use a precoderin accordance with a specific resource. For instance, in case of usingthe precoder according to the method of Formula 9, a method of applyinga precoder in accordance with a specific resource may be represented asFormula 15.P _(N) _(t) _(×R,n,m)(k)=W _(k mod n) _(c) ⊗P _(k mod m) _(c)

In Formula 15, k indicates a specific resource region. A precodingmatrix for a specific resource region k is determined by such a modulooperation as Formula 15. In this case, n_(c) and m_(c) may indicate asize or subset of a codebook for matrix W and a size or subset of acodebook for a matrix P, respectively.

Like Formula 15, if cycling is applied to each of the two matrixes,complexity may increase despite maximizing a diversity gain. Hence,long-term cycling may be set to be applied to a specific matrix andshort-term cycling may be set to be applied to the rest of the matrixes.

For instance, the matrix W may be configured to perform a modulooperation in accordance with a physical resource block (PRB) index andthe matrix P may be configured to perform a modulo operation inaccordance with a subframe index. Alternatively, the matrix W may beconfigured to perform a modulo operation in accordance with a subframeindex and the matrix P may be configured to perform a modulo operationin accordance with a physical resource block (PRB) index.

For another instance, the matrix W may be configured to perform a modulooperation in accordance with a physical resource block (PRB) index andthe matrix P may be configured to perform a modulo operation inaccordance with a subband index. Alternatively, the matrix W may beconfigured to perform a modulo operation in accordance with a subbandindex and the matrix P may be configured to perform a modulo operationin accordance with a physical resource block (PRB) index.

Moreover, a precoder cycling using a modulo operation is applied to oneof the two matrixes only and the other may be fixed to use.

Codebook Configuration for 8 Tx Antennas

In the 3GPP LTE Release-10 system having an extended antennaconfiguration (e.g., 8 Tx antennas), the feedback scheme used by thelegacy 3GPP LTE Release-8/9 may be applied in a manner of beingextended. For instance, it is able to feed back such channel stateinformation (CSI) as RI (Rank Indicator), PMI (Precoding Matrix Index),CQI (Channel Quality Information) and the like. In the followingdescription, a method of designing a dual precoder based feedbackcodebook usable for a system supportive of an extended antennaconfiguration is explained. In the dual precoder based feedbackcodebook, in order to indicate a precoder to be used for MIMOtransmission of a transmitting end, a receiving end may be able totransmit a precoding matrix index (PMI) to the transmitting end. Indoing so, a precoding matrix may be indicated by combination of 2different PMIs. In particular, the receiving end feeds back 2 differentPMIs (i.e., 1^(st) PMI and 2^(nd) PM) to the transmitting end.Subsequently, the transmitting end determines the precoding matrixindicated by the combination of the 1^(st) and 2^(nd) PMIs and is thenable to apply the determined precoding matrix to the MIMO transmission.

IN designing the dual precoder based feedback codebook, it may be ableto consider 8-Tx antenna MIMO transmission, single user-MIMO (SU-MIMO)and multiple user-MIMO (MU-MIMO) supports, compatibility with variousantenna configurations, codebook design references, codebook size andthe like.

As a codebook applicable to 8-Tx antenna MIMO transmission, it may beable to consider designing a feedback codebook. In particular, thisfeedback codebook supports SU-MIMO only in case of a rank higher than 2,is optimized for both SU-MIMO and MU-MIMO in case of a rank equal to orlower than 2, and is compatible with various antenna configurations.

In case of MU-MIMO, user equipments participating in MU-MIMO arepreferably separated in correlation domain. Hence, the codebook forMU-MIMO needs to be designed to correctly operate on a channel havinghigh correlation. Since DFT vectors provide good performance on achannel having high correlation, it may be able to consider having DFTvector contained in a codebook set of a rank up to a rank-2. In highscattering propagation environment (e.g., an indoor environment havingmany reflective waves, etc.) capable of producing many space channels,SU-MIMO operation may be preferred as the MIMO transmission scheme.Hence, it may be able to consider designing a codebook for a rank higherthan the rank-2 to have god performance in separating multiple layers.

In designing a precoder for MIMO transmission, it is preferable that oneprecoder structure has good performance for various antennaconfigurations (e.g., low-correlation, high-correlation, cross-pole,etc.). In arrangement of 8 Tx antennas, a cross-polarized array havingan antenna interval of 4λ may be formed in a low-correlation antennaconfiguration, a ULA having an antenna interval of 0.5λ may be formed ina high-correlation antenna configuration, or a cross-polarized arrayhaving an antenna interval of 0.5λ may be formed in a cross-polarizedantenna configuration. The DFT based codebook structure may be able toprovide good performance for the high-correlation antenna configuration.Meanwhile, block diagonal matrixes may be more suitable for thecross-polarized antenna configuration. Hence, in case that a diagonalmatrix is introduced into a codebook for 8 Tx antennas, it is able toconfigure a codebook that provides god performance for all antennaconfigurations.

As mentioned in the foregoing description, the codebook design referenceis to meet the unitary codebook, the CM property, the constrainedalphabet, the proper codebook size, the nested property and the like.This applies to the 3GPP LTE Release-8/9 codebook design. And, it may beable to consider applying such a codebook design reference to the 3GPPLTE Release-10 codebook design supportive of the extended antennaconfiguration.

Regarding the codebook size, it is necessary to increase the codebooksize to sufficiently support the advantage in using 8 Tx antennas. Inorder to obtain a sufficient precoding gain from 8 Tx antennas inenvironment with low correlation, a codebook in large size (e.g., acodebook in size over 4 bits for Rank 1 or Rank 2) may be required. Inorder to obtain a precoding gain in an environment with highcorrelation, a codebook in O-bit size may be sufficient. Yet, in orderto achieve a multiplexing gain of MU-MIMO, it may be able to increase acodebook size for Rank 1 or Rank 2.

Based on the above description, general structures of a codebook for 8Tx antennas are explained as follows.

Codebook Structure (1)

In applying multi-granular feedback, a method of configuring a codebookfor 8 Tx antennas by combination of 2 base matrixes and a method ofconfiguring the combination of 2 base matrixes using an inner productare described as follows.

First of all, a method of using an inner product of 2 base matrixes maybe represented as Formula 16.W={tilde over (W)} ₁ {tilde over (W)} ₂  [Formula 16]

In case that codebook for 8 Tx antennas is represented in form of aninner product, a 1^(st) base matrix may be represented as a diagonalmatrix shown in Formula 17 for a co-polarized antenna group.

$\begin{matrix}{{\overset{\sim}{W}}_{1} = {\begin{bmatrix}W_{1} & 0 \\0 & W_{1}\end{bmatrix}\left( {W_{1}\text{:}\mspace{14mu} 4 \times N} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 17} \right\rbrack\end{matrix}$

Moreover, in case that a 2^(nd) base matrix is used to adjust a relativephase between polarizations, the 2^(nd) base matrix may be representedusing an identity matrix. For an upper rank of a codebook for 8 Txantennas, the 2^(nd) base matrix may be represented as Formula 18. InFormula 18, a relation between a coefficient ‘1’ of a 1^(st) row of the2^(nd) base matrix and a coefficient ‘a’ or ‘−a’ of a 2^(nd) row thereofmay be able to reflect the adjustment of a relative phase betweenorthogonal polarizations.

$\begin{matrix}{{\overset{\sim}{W}}_{2} = {\begin{bmatrix}I & I \\{aI} & {- {aI}}\end{bmatrix}\left( {I\text{:}\mspace{14mu} N \times N} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 18} \right\rbrack\end{matrix}$

Hence, if the codebook for 8 Tx antennas is represented using the 1^(st)base matrix and the 2^(nd) base matrix, it can be represented as Formula19.

$\begin{matrix}{W = {{\begin{bmatrix}W_{1} & 0 \\0 & W_{1}\end{bmatrix}\begin{bmatrix}I & I \\{aI} & {- {aI}}\end{bmatrix}} = \begin{bmatrix}W_{1} & W_{1} \\{aW}_{1} & {- {aW}_{1}}\end{bmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 19} \right\rbrack\end{matrix}$

The codebook expressed using the inner product like Formula 19 can besimplified into Formula 20 using Kroneker product.W=W ₂ ⊗W ₁(W ₁:4×N,W ₂:2×M)  [Formula 20]

In Formula 20, a precoding matrix included in a codebook W includes 4*2rows and N*M columns. Hence, it can be used as a codebook for 8 Txantennas and transmission of Rank ‘N*M’. For instance, in case ofconfiguring a codebook for 8 Tx antennas and transmission of Rank R, ifW₂ is configured with 2×M, a value N for W₁ becomes R/M. For instance,in case of configuring a codebook for 8 Tx antennas and transmission ofRank 4, if W₂ is configured with 2×2 (i.e., M=2) matrix (e.g., thematrix shown in Formula 13), W₁ may apply 4×2 (i.e., N=R/M=4/2=2) matrix(e.g., DFT matrix).

Codebook Structure (2)

Another method of configuring a codebook for 8 Tx antennas bycombination of 2 base matrixes is described as follows. Assuming thatthe 2 base matrixes are set to W1 and W2, respectively, a precodingmatrix W for configuring a codebook may be defined in form of W1*W2. ForRank 1 to Rank 8, W1 may be able to have such a form of a block diagonalmatrix as

$\begin{bmatrix}X & 0 \\0 & X\end{bmatrix}.$

For Rank 1 to Rank 4, X corresponding to a block of a block diagonalmatrix W 1 may be configured with a matrix in size of 4×Nb. And, 16 4TxDFT beams can be defined for the X. In this case, beams indexes may begiven as 0, 1, 2, . . . , and 15, respectively. For each W1, theadjacent overlapping beams may be usable to reduce an edge effect infrequency-selective precoding. Hence, even if a codebook is configuredusing the same W1 for an identical or different W2, optimal performancecan be secured for several subbands.

For Rank 1 and Rank 2, X corresponding to a block diagonal matrix W1 maybe configured with a matrix in size of 4×4 (i.e., Nb=4). For each ofRank 1 and Rank 2, 8 W1 matrixes can be defined. And, one W1 may includebeams overlapping with the adjacent W1. In case that beam indexes aregiven as 0, 1, 2, . . . , and 15, respectively, for example, it is ableto configure 8 W1 matrixes, of which beams overlapping with the adjacentW1 matrix, such as {0, 1, 2, 3}, {2, 3, 4, 5}, {4, 5, 6, 7}, {6, 7, 8,9},{8, 9, 10, 11}, {10, 11, 12, 13}, {12, 13, 14, 15}, and {14, 15, 0,1}. For instance, a W1 codebook for Rank 1 or Rank 2 may be defined asFormula 21.

$\begin{matrix}{X^{(n)} = {\frac{1}{2} \times \left\lbrack \begin{matrix}1 & 0 & 0 & 0 \\0 & e^{j\frac{\pi}{4}n} & 0 & 0 \\0 & 0 & e^{{j{(2)}}\frac{\pi}{4}n} & 0 \\0 & 0 & 0 & e^{{j{(3)}}\frac{\pi}{4}n}\end{matrix} \right\rbrack{\quad{\left\lbrack \begin{matrix}1 & 1 & 1 & 1 \\1 & e^{j\frac{\pi}{8}} & e^{{j{(2)}}\frac{\pi}{8}} & e^{{j{(3)}}\frac{\pi}{8}} \\1 & e^{{j{(2)}}\frac{\pi}{8}} & e^{{j{(2)}}{(2)}\frac{\pi}{8}} & e^{{j{(3)}}{(2)}\frac{\pi}{8}} \\1 & e^{{j{(3)}}\frac{\pi}{8}} & e^{{j{(2)}}{(3)}\frac{\pi}{8}} & e^{{j{(3)}}{(3)}\frac{\pi}{8}}\end{matrix} \right\rbrack,\mspace{20mu}{n = 0},1,2,\ldots\mspace{14mu},{{7\mspace{20mu} W_{1}^{(n)}} = \begin{bmatrix}X^{(n)} & 0 \\0 & X^{(n)}\end{bmatrix}},\mspace{20mu}{{CB}_{1} = \left\{ {W_{1}^{(0)},W_{1}^{(1)},W_{1}^{(2)},\ldots\mspace{14mu},W_{1}^{(7)}} \right\}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 21} \right\rbrack\end{matrix}$

In Formula 21, X(n) corresponding to a block of a block diagonal matrixW1 ^((n)) is defined and a W1 codebook (CB₁) can be configured with 8different W1's.

Considering the selection and common-phase component of W2, theselection of 4 kinds of different matrixes is possible for Rank 1 and 4kinds of different QPSK co-phases are applicable for Rank 1. Hence,total 16 W2 matrixes can be defined. For instance, the W2 codebook (CB₂)for Rank 1 can be configured as Formula 22.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\Y\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{j\; Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{- Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{- j}\; Y}\end{bmatrix}}} \right\}}\mspace{20mu}{Y \in \left\{ {\begin{bmatrix}1 \\0 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\1 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\1 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\0 \\1\end{bmatrix}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 22} \right\rbrack\end{matrix}$

For Rank 2, the selection of 4 kinds of different matrixes is possibleand 2 kinds of different QPSK co-phases are applicable. Hence, total 8W2 matrixes can be defined. For instance, the W2 codebook (CB₂) for Rank2 can be configured as Formula 23.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y & Y \\Y & {- Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y & Y \\{j\; Y} & {{- j}\; Y}\end{bmatrix}}} \right\}}{Y \in \left\{ {\begin{bmatrix}1 \\0 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\1 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\1 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\0 \\1\end{bmatrix}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 23} \right\rbrack\end{matrix}$

Subsequently, for Rank 3 and Rank 4, X corresponding to a block diagonalmatrix W1 may be configured with a matrix in size of 4×8 (i.e., Nb=8).For each of Rank 3 and Rank 4, 4 W1 matrixes can be defined. And, one W1may include beams overlapping with the adjacent W1. In case that beamindexes are given as 0, 1, 2, . . . , and 15, respectively, for example,it is able to configure 4 W1 matrixes, of which beams overlapping withthe adjacent W1 matrix, such as {0, 1, 2, . . . , 7}, {4, 5, 6, . . . ,11}, {8, 9, 10, . . . , 15}, and {12, . . . , 15, 0, . . . , 3}. Forinstance, a W1 codebook for Rank 3 or Rank 4 may be defined as Formula24.

$\begin{matrix}{X^{(n)} = {\frac{1}{2} \times \left\lbrack \begin{matrix}1 & 0 & 0 & 0 \\0 & (j)^{n} & 0 & 0 \\0 & 0 & \left( {- 1} \right)^{n} & 0 \\0 & 0 & 0 & \left( {- j} \right)^{n}\end{matrix} \right\rbrack{\quad{\left\lbrack \begin{matrix}1 & 1 & 1 & \ldots & 1 \\1 & {\mathbb{e}}^{j\frac{\pi}{8}} & {\mathbb{e}}^{{j{(2)}}\frac{\pi}{8}} & \ldots & {\mathbb{e}}^{{j{(7)}}\frac{\pi}{8}} \\1 & {\mathbb{e}}^{{j{(2)}}\frac{\pi}{8}} & {\mathbb{e}}^{{j{(2)}}{(2)}\frac{\pi}{8}} & \ldots & {\mathbb{e}}^{{j{(7)}}{(2)}\frac{\pi}{8}} \\1 & {\mathbb{e}}^{{j{(3)}}\frac{\pi}{8}} & {\mathbb{e}}^{{j{(2)}}{(3)}\frac{\pi}{8}} & \ldots & {\mathbb{e}}^{{j{(7)}}{(3)}\frac{\pi}{8}}\end{matrix} \right\rbrack,\mspace{20mu}{n = 0},1,2,{{3\mspace{20mu} W_{1}^{(n)}} = \begin{bmatrix}X^{(n)} & 0 \\0 & X^{(n)}\end{bmatrix}},\mspace{20mu}{{CB}_{1} = \left\{ {W_{1}^{(0)},W_{1}^{(1)},W_{1}^{(2)},W_{1}^{(3)}} \right\}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 24} \right\rbrack\end{matrix}$

In Formula 24, X(n) corresponding to a block of a block diagonal matrixW1 ^((n)) is defined and a W1 codebook (CB₁) can be configured with 4different W1's.

Considering the selection and common-phase component of W2, theselection of 8 kinds of different matrixes is possible for Rank 3 and 2kinds of different QPSK co-phases are applicable for Rank 3. Hence,total 16 W2 matrixes can be defined. For instance, the W2 codebook (CB₂)for Rank 3 can be configured as Formula 25.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & {- Y_{2}}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y_{1} & Y_{2} \\{jY}_{1} & {- {jY}_{2}}\end{bmatrix}}} \right\}}{\left( {Y_{1},Y_{2}} \right) \in {\quad{\quad\begin{Bmatrix}\begin{matrix}{\left( {e_{1},\begin{bmatrix}e_{1} & e_{5}\end{bmatrix}} \right),\left( {e_{2},\begin{bmatrix}e_{2} & e_{6}\end{bmatrix}} \right),} \\{\left( {e_{3},\begin{bmatrix}e_{3} & e_{7}\end{bmatrix}} \right),\left( {e_{4},\begin{bmatrix}e_{4} & e_{8}\end{bmatrix}} \right),}\end{matrix} \\\begin{matrix}{\left( {e_{5},\begin{bmatrix}e_{1} & e_{5}\end{bmatrix}} \right),\left( {e_{6},\begin{bmatrix}e_{2} & e_{6}\end{bmatrix}} \right),} \\{\left( {e_{7},\begin{bmatrix}e_{3} & e_{7}\end{bmatrix}} \right),\left( {e_{8},\begin{bmatrix}e_{4} & e_{8}\end{bmatrix}} \right)}\end{matrix}\end{Bmatrix}}}}} & \left\lbrack {{Formula}\mspace{14mu} 25} \right\rbrack\end{matrix}$

In Formula 24, e_(n), indicates 8×1 vector, n^(th) element has a valueof 1, and the rest of elements mean a selection vector having a value of0.

For Rank 4, the selection of 4 kinds of different matrixes is possibleand 2 kinds of different QPSK co-phases are applicable. Hence, total 8W2 matrixes can be defined. For instance, the W2 codebook (CB₂) for Rank4 can be configured as Formula 26.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y & Y \\Y & {- Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y & Y \\{jY} & {- {jY}}\end{bmatrix}}} \right\}}{Y \in \left\{ {\begin{bmatrix}e_{1} & e_{5}\end{bmatrix},\begin{bmatrix}e_{2} & e_{6}\end{bmatrix},\begin{bmatrix}e_{3} & e_{7}\end{bmatrix},\begin{bmatrix}e_{4} & e_{8}\end{bmatrix}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 26} \right\rbrack\end{matrix}$

For Rank 5 to Rank 8, X corresponding to a block of a block diagonalmatrix W1 can be configured with DFT matrix in size of 4×4 and one W1matrix can be defined. W2 may be defined as a product of a matrix

$\begin{bmatrix}I & I \\I & {- I}\end{bmatrix}\quad$and a row selection matrix in a fixed size of 8×r. For Rank 5, sinceselection of 4 kinds of different matrixes is possible, 4 W2 matrixescan be defined. For Rank 6, since selection of 4 kinds of differentmatrixes is possible, 4 W2 matrixes can be defined. For Rank 7, sinceselection of 1 kind of a matrix is possible, one W2 matrix can bedefined. For Rank 8, since selection of 1 kind of a matrix is possible,one W2 matrix can be defined. In this case, the matrix

$\begin{bmatrix}I & I \\I & {- I}\end{bmatrix}\quad$is introduced to enable all polarized groups for each transmission layerto be identically used and good performance may be expected for atransmission of a high rank having a spatial channel having morescattering. In this case, the I means an identity matrix.

For instance, the W1 codebook or the W2 codebook for Rank 5 to Rank 8can be defined as Formula 27.

$\begin{matrix}{{{X = {\frac{1}{2} \times \begin{bmatrix}1 & 1 & 1 & 1 \\1 & j & {- 1} & {- j} \\1 & {- 1} & 1 & {- 1} \\1 & {- j} & {- 1} & j\end{bmatrix}}},{W_{1} = \begin{bmatrix}X & 0 \\0 & X\end{bmatrix}},{{CB}_{1} = \left\{ W_{1} \right\}}}{W_{2} \in {{CB}_{2}\left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}I_{4} & I_{4} \\I_{4} & {- I_{4}}\end{bmatrix}}Y} \right\}}}} & \left\lbrack {{Formula}\mspace{14mu} 27} \right\rbrack\end{matrix}$

In Formula 27, the W1 codebook for Rank 5 to Rank 8 is configured withone W1 matrix only. I₄ in the W2 codebook for Rank 5 to Rank 8 means anidentity matrix in size of 4×4. In Formula 27, a matrix Y can be definedas one of Formula 28 to Formula 31 for example.

The matrix Y for Rank 5 can be defined as Formula 28.

$\begin{matrix}{Y \in \begin{Bmatrix}{\begin{bmatrix}e_{1} & e_{2} & e_{3} & e_{4} & e_{5}\end{bmatrix},\begin{bmatrix}e_{2} & e_{3} & e_{4} & e_{5} & e_{6}\end{bmatrix},} \\{\begin{bmatrix}e_{3} & e_{4} & e_{5} & e_{6} & e_{7}\end{bmatrix},\begin{bmatrix}e_{4} & e_{5} & e_{6} & e_{7} & e_{8}\end{bmatrix},}\end{Bmatrix}} & \left\lbrack {{Formula}\mspace{14mu} 28} \right\rbrack\end{matrix}$

The matrix Y for Rank 6 can be defined as Formula 29.

$\begin{matrix}{{{Y \in}\quad}{\quad{\begin{Bmatrix}\begin{matrix}{\begin{bmatrix}e_{1} & e_{2} & e_{3} & e_{4} & e_{5} & e_{6}\end{bmatrix},} \\{\begin{bmatrix}e_{2} & e_{3} & e_{4} & e_{5} & e_{6} & e_{7}\end{bmatrix},}\end{matrix} \\\begin{matrix}{\begin{bmatrix}e_{3} & e_{4} & e_{5} & e_{6} & e_{7} & e_{8}\end{bmatrix},} \\{\begin{bmatrix}e_{4} & e_{5} & e_{6} & e_{7} & e_{8} & e_{1}\end{bmatrix},}\end{matrix}\end{Bmatrix}{\quad\quad}}}} & \left\lbrack {{Formula}\mspace{14mu} 29} \right\rbrack\end{matrix}$

The matrix Y for Rank 7 can be defined as Formula 30.

[Formula 30]Y=[e ₁ e ₂ e ₃ e ₄ e ₅ e ₆ e ₇]

The matrix Y for Rank 8 can be defined as Formula 31Y=I ₈  [Formula 31]

In Formula 31, the I₈ means 8×8 identity matrix.

As mentioned in the foregoing description, the numbers of W1's, whichcan be defined for each of Rank 1 to Rank 8, are added up to result in28 (=8+8+4+4+1+1+1+1).

Based on the above-mentioned description, proposed is a method ofreducing the number of reference signal (RS) ports, for informing areceiving end of a channel to which precoding is applied, below thenumber of transmitted layers by calculating a block matrix based on aprecoding matrix for antennas in a horizontal direction of and aprecoding matrix for antennas in a vertical direction at a transmittingend in 3D MIMO system having 2-dimensional active antenna system(2D-AAS) proposed in the present invention installed therein. Forinstance, if final precoding in the form of Kronecker product ofprecoding for the antennas in the horizontal direction and precoding forthe antennas in the vertical direction is used at the transmitting endin the 2D-AAS installed 3D MIMO system, the number of the referencesignal (RS) ports for informing the channel to which the precoding isapplied can be reduced.

Referring to FIG. 24, an antenna system that utilizes AAS is described.After LTE Rel-12, the antenna system utilizing the AAS shown in FIG. 24has been discussed. Since each antenna in the AAS corresponds to anactive antenna including an active circuit, an antenna pattern can bechanged depending on a situation. Thus, it is more efficient in reducinginterference or performing beamforming. Moreover, if the AAS isestablished in two dimensions (i.e., 2D-AAS), in aspect of the antennapattern, a main lobe of the antenna is adjusted more efficiently in 3dimensions. Thus, it is possible to actively change a transmitting beamdepending on a location of the receiving end. Accordingly, the 2D-AASmay be established as a system having multiple antennas in a manner ofarranging antennas vertically and horizontally as shown in FIG. 24.

When the 2D-AAS is introduced, for the efficient use of antennas, areceiving end should provide a transmitting end with feedback of CSI(channel state information), which is channel information between thetransmitting end and the receiving end. However, in the 2D-AAS, a numberof antennas may be used in both of the transmitting end and receivingend. Moreover, the amount of channel information, which needs to be fedback by the receiving end, may increase as the number of antennasincreases. For instance, if 4 bits of PMI is required for 4 transmittingantennas, in case that 64 antennas are implemented using 2D-AAS, it isgenerally expected that 64 bits of PMI is required. However,transmission of such a large amount of channel information is not onlyefficient but also causes complexity, which makes the receiving endimpossible to handle the amount of PMI, CQI and RI calculations withrespect to CSI within a limited time.

In order to solve the problems of the feedback in the 2D-AAS and thecomplexity in the receiving end, a precoding scheme having a structureof the Kronecker product shown in Formula 32 may be applied. In thepresent invention, V indicating a vertical direction may beinterchangeably used with H indicating a horizontal direction.PM=[W _(V) ⁽¹⁾ ⊗W _(H) ⁽¹⁾ W _(V) ⁽²⁾ ⊗W _(H) ⁽²⁾ . . . W _(V) ^((i)) ⊗W_(H) ^((i)) . . . W _(V) ^((N) ^(P) ⁾ ⊗W _(H) ^((N) ^(P) ^()])  [Formula32]

Formula 32 indicates final precoding in the form of combining precodingsof N_(P) Kronecker product (KP) structures with index i. ⊗ means KPoperation. In Formula 32, W_(V) ^((i)) represents precoding for antennaelements (or ports) in the vertical direction in i^(th) KP structure ofprecoding and W_(H) ^((i)) represents precoding for antenna elements (orports) in the horizontal direction in i^(th) KP structure of precoding.A size of W_(V) ^((i)) is N_(V)-by-r_(V) ^((i)) and a size of W_(H)^((i)) is N_(H)-by-r_(H) ^((i)). N_(V) and N_(H) indicate the number ofantenna elements (or ports) in the vertical direction and the number ofantenna elements (or ports) in the horizontal direction, respectively.r_(V) ^((i)) and r_(H) ^((i)) indicate the number of layers in thevertical direction corresponding to the i^(th) KP structure of precodingand the number of layers in the horizontal direction corresponding tothe i^(th) KP structure of precoding, respectively. Therefore, the totalnumber of layers corresponding to respective KP structures of precodingsmay be expressed as r_(V) ^((i))×r_(H) ^((i)). And, the total number oflayers in Formula 32 may be expressed as

$\sum\limits_{i}{r_{V}^{(i)} \times {r_{H}^{(i)}.}}$

Moreover, if a value of the N_(P) decreases, it may be checked thatfeedback is performed more efficiently. For instance, if N_(P)=1, whileN_(V)=N_(H)=8, conventional precoding should be selected from codebookhaving 64-by-1 size of codeword to transmit a single layer. However, inprecoding according to Formula 32, two precodings may be selected fromcodebook having 8-by-1 size of codeword. Therefore, it may be checkedthat implementation can be accomplished with a small amount of feedbackand low complexity owing to difference between sizes of codebooks.

Thus, proposed in the present invention is a method of reducing thenumber of reference signal ports below the total number of transmittedlayers in case that a transmitting end transmits a reference signal (RS)(e.g., DM-RS used in current LTE) for informing a precoding-appliedchannel to a receiving end in a system using KP-based precoding (i.e.,Formula 32).

In particular, in the current LTE, a transmitting end uses differentprecoding instead of precoding fed back from a receiving end and usesDM-RS to inform a channel to which the corresponding precoding isapplied, for unrestricted use of precoding. Through the DM-RS receivedfrom the transmitting end, the receiving end may implicitly know howdata of the receiving end is precoded. And, the receiving end performsdemodulation using the DM-RS. In particular, the present inventionrelates to a technology for the channel to which precoding is applied.Although the present invention is described with reference to DM-RS forthe convenience of the explanation, it is not limited to the DM-RS,which is applied to the legacy wireless communication system (e.g., LTEsystem or system before LTE-A release 11).

For the convenience of the explanation, it is assumed in the presentinvention that a channel from 2D-AAS to a receiving end is representedas the form of the Kronecker product as shown in Formula 33.

$\begin{matrix}{H = {\begin{bmatrix}H_{T}^{(1)} \\H_{T}^{(2)} \\\vdots \\H_{T}^{(j)} \\\vdots \\H_{T}^{(N_{R})}\end{bmatrix} = \begin{bmatrix}{H_{V}^{(1)} \otimes H_{H}^{(1)}} \\{H_{V}^{(2)} \otimes H_{H}^{(2)}} \\\vdots \\{H_{V}^{(j)} \otimes H_{H}^{(j)}} \\\vdots \\{H_{V}^{(N_{R})} \otimes H_{H}^{(N_{R})}}\end{bmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 33} \right\rbrack\end{matrix}$

In Formula 33, H means entire channels from a transmitting end to a)receiving end and H_(T) ^((j)) means a channel from the transmitting endto j^(th) receiving antenna. H_(V) ^((j)) and H_(H) ^((j)) mean achannel from an antenna element (or port) in the vertical direction toj^(th) antenna in the receiving end and a channel from an antennaelement (or port) in the horizontal direction to the j^(th) antenna inthe receiving end, respectively. In FIG. 24, it is assumed that H_(V)^((j)) exists only at antennas in a vertical directional block (i.e., Ablock) and it means a channel from an antenna in the A block to thej^(th) antenna in the receiving end. And, it is assumed that H_(H)^((j)) exists at antennas in a horizontal directional block (i.e., Bblock) and it means a channel from an antenna in the B block to thej^(th) antenna in the receiving end.

For the convenience of the explanation, the present invention isdescribed on the assumption that there is one random receiving antenna.Moreover, in case that the present invention is extensively applied toat least one different receiving antenna, it will be additionallyexplained.

Embodiments of the Present Invention with Respect to One RandomReceiving Antenna

For the convenience of the explanation, the present embodiments aredescribed using only a channel from a transmitting end to one randomreceiving antenna without index (j) as shown in Formula 34. Formula 34in the following description is only to explain the present invention.In particular, the present invention can be applied even though anactual channel is unlike Formula 34.H _(T) =H _(V) ⊗H _(H)  [Formula 34]

In particular, in case of N_(P)=1 in Formula 32, a precoding matrix maybe expressed as Formula 35.PM=W _(V) ⁽¹⁾ ⊗W _(H) ⁽¹⁾  [Formula 35]

The purpose of DM-RS port is to inform precoding related data to whichthe precoding is applied. Thus, the receiving end needs to obtain aproduct of a channel and precoding through the DM-RS port. In this case,the product of the channel and the precoding may be expressed as Formula36 using Formula 34 and Formula 35.

$\begin{matrix}{\begin{matrix}{{H_{T} \times {PM}} = {\left( {H_{V} \otimes H_{H}} \right) \times \left( {W_{V}^{(1)} \otimes W_{H}^{(1)}} \right)}} \\{= {\left( {H_{V}W_{V}^{(1)}} \right) \otimes \left( {H_{H}W_{H}^{(1)}} \right)}}\end{matrix}\quad} & \left\lbrack {{Formula}\mspace{14mu} 36} \right\rbrack\end{matrix}$In Formula 36, (H_(V)⊗H_(H))×(W_(V) ⁽¹⁾⊗W_(H) ⁽¹⁾) may be substitutedwith (H_(V)W_(V) ⁽¹⁾)⊗(H_(H)W_(H) ⁽¹⁾) according to the features of theKronecker product (KP) operation.

When DM-RS is transmitted in the legacy wireless communication system,DM-RS ports as many as the number of layers included in PM should beused to inform the receiving end of H_(T)×PM. In particular, if PMcorresponds to precoding selected for transmission of 16 layers, 16DM-RS ports to which PM is applied should be used.

According to the present invention, channels H_(V)W_(V) ⁽¹⁾ andH_(H)W_(H) ⁽¹⁾ as shown in Formula 36 may be informed the receiving endfor the purpose of reduction in overhead of DM-RS. Thus, the receivingend can restore H_(T)×PM using the received channels H_(V)W_(V) ⁽¹⁾ andH_(H)W_(H) ⁽¹⁾. In the following description, two representativeembodiments are explained.

According to a 1^(st) embodiment, r_(V) ⁽¹⁾ of DM-RS ports (hereinafterreferred to as V-DM-RS) where W_(V) ⁽¹⁾ precoding is applied to antennasof a specific column (e.g., 1^(st) column) in the vertical direction andr_(H) ⁽¹⁾ of DM-RS ports (hereinafter referred to as II-DM-RS) whereW_(H) ⁽¹⁾ precoding is applied to antennas of a specific row (e.g.,1^(st) row) in the horizontal direction may be transmitted together.

Thus, in each receiving antenna, the receiving end may restore a channelrequired in demodulation through the Kronecker product of channelH_(V)W_(V) ⁽¹⁾ estimated through V-DM-RS and channel H_(H)W_(H) ⁽¹⁾estimated through H-DM-RS.

In case that either r_(v) ⁽¹⁾ or r_(H) ⁽¹⁾ is 1, for example, if r_(V)⁽¹⁾ is 1, only H-DM-RS having r_(H) ⁽¹⁾ ports is used. And, if the r_(H)⁽¹⁾ is 1, only V-DM-RS having r_(V) ⁽¹⁾ ports is used. In this case,DM-RS applies W_(V) ⁽¹⁾⊗W_(H) ⁽¹⁾ precoding to whole antennas in both ofthe vertical and horizontal directions. If both r_(V) ⁽¹⁾ and r_(H) ⁽¹⁾are 1, the number of DM-RS ports becomes 1. In this case, DM-RS appliesthe W_(V) ⁽¹⁾⊗W_(H) ⁽¹⁾ precoding. Therefore, it is preferred to applythe present invention to the case that either r_(V) ⁽¹⁾ or r_(H) ⁽¹⁾ isequal to or greater than 2.

Therefore, according the present invention, overhead of DM-RS can bereduced compared to the conventional DM-RS transmission method. Forexample, in case of a system with r_(V) ⁽¹⁾=r_(H) ⁽¹⁾=8 and N_(P)=1, 64DM-RSs are required for 64 layers. However, according to the presentinvention, the system can be implemented using only 16 (i.e., r_(V)⁽¹⁾+r_(H) ⁽¹⁾) DM-RS ports.

According to a 2^(nd) embodiment, among whole antennas in both of thevertical and horizontal directions, V-DM-RS ports corresponding toantennas in the vertical direction and H-DM-RS ports corresponding toantennas in the horizontal direction are simultaneously transmitted.Moreover, DM-RS ports for antennas included in both of the antennas inthe vertical direction and the antennas in the horizontal direction maybe configured to be transmitted once.

For instance, r_(H) ⁽¹⁾ of H-DM-RS ports where W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗W_(H)⁽¹⁾ precoding is applied to the whole antennas in both of the verticaland horizontal directions and (r_(V) ⁽¹⁾−1) of V-DM-RS ports where W_(V)⁽¹⁾(:,˜k⁽¹⁾)⊗W_(H) ⁽¹⁾(:,l⁽¹⁾) precoding is applied to the wholeantennas in both of the vertical and horizontal directions may betransmitted together. As another example, r_(V) ⁽¹⁾ of V-DM-RS portswhere W_(V) ⁽¹⁾⊗W_(H) ⁽¹⁾(:,l⁽¹⁾) precoding is applied to the wholeantennas in both of the vertical and horizontal directions and (r_(H)⁽¹⁾−1) of H-DM-RS ports where W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗W_(H) ⁽¹⁾(:,˜l⁽¹⁾)precoding is applied to the whole antennas in both of the vertical andhorizontal directions may be transmitted together. In theabove-mentioned precoding-related formula, A(:,a) means a^(th) column inA matrix and A(:,˜a) means a remaining matrix except the a^(th) columnin the A matrix.

According to the 2^(nd) embodiment, the total number of DM-RS portsbecomes ‘r_(H) ⁽¹⁾+r_(V) ⁽¹⁾−1’. Compared to r_(V) ⁽¹⁾r_(H) ⁽¹⁾ of DM-RSports in the related art, the total number of DM-RS ports is reduced.Moreover, compared to ‘r_(H) ⁽¹⁾+r_(V) ⁽¹⁾’ of DM-RS ports in the 1^(st)embodiment, one DM-RS port can be reduced. In the 2^(nd) embodiment,indices k⁽¹⁾ and l⁽¹⁾ may be previously defined between the transmittingand receiving ends or be configured through RRC signaling. The receivingend may restore a channel using a channel estimated through receivedV-DM-RS and H-DM RS in each receiving antenna.

FIG. 25 is a reference diagram to describe a method for restoring entirechannels at a receiving end according to the present invention.According to the 2^(nd) embodiment with reference to FIG. 25, areceiving end may measure H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾ channelthrough H-DM-RS and measure H_(V)W_(V) ⁽¹⁾(:,˜k⁽¹⁾)⊗H_(H)W_(H)⁽¹⁾(:,l⁽¹⁾) channel through V-DM-RS. In particular, a channel formed bythe Kronecker product of H_(V)W_(V) ⁽¹⁾ and H_(H)W_(H) ⁽¹⁾ in Formula 36can be assumed to be a channel from one virtual antenna with a verticalantenna and a horizontal antenna. (This mapping may be consideredsimilar to that the channel from the antenna to the receiving end shownin FIG. 24 is expressed as Formula 35. However, the mapping serves toexplain the present invention and is not construed to limit the presentinvention.)

In this case, if there is only the vertical direction of a virtualantenna(s), H_(V)W_(V) ⁽¹⁾ is defined as a channel from the virtualantenna(s). And, if there is only the horizontal direction of a virtualantenna(s), H_(H)W_(H) ⁽¹⁾ is defined as a channel from the virtualantenna(s). Through the above process, the receiving end may know whichvirtual antenna(s) from the channel is currently measured, as shown inFIG. 25.

FIG. 25 illustrates the case that r_(V) ⁽¹⁾=r_(H) ⁽¹⁾=4, k⁽¹⁾=2, andl⁽¹⁾=3. In this case, if it is defined that H_(V)W_(V) ⁽¹⁾=[α₁ α₂ α₃ α₄]and H_(H)W_(H) ⁽¹⁾=[β₁ β₂ β₃ β₄], a value of a channel from each of thevirtual antennas to a specific receiving antenna may be expressed asFIG. 25. As can be seen in the drawing, entire channels can be restoredbased on only currently measured channels.

In particular, since k⁽¹⁾=2, H_(V)W_(V) ⁽¹⁾(:, 2)⊗H_(H)W_(H) ⁽¹⁾ channelis measured through H-DM-RS. And, since k⁽¹⁾=2 and l⁽¹⁾=3, H_(V)W_(V)⁽¹⁾(:,˜2)⊗H_(H)W_(H) ⁽¹⁾(:, 3) channel is measured through V-DM-RS.Thus, the entire channels ((H_(V)W_(V) ⁽¹⁾)⊗(H_(H)W_(H) ⁽¹⁾), i.e.,4-by-4 size of channels) may be restored.

More particularly, values of H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾ andH_(V)W_(V) ⁽¹⁾(:,˜k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) are measured as mentionedin the foregoing description. The receiving end calculates H_(V)W_(V)⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) by adding l⁽¹⁾th value of H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾ between (k⁽¹⁾−1)th value and k⁽¹⁾th value ofH_(V)W_(V) ⁽¹⁾(:,˜k⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾). Subsequently, thereceiving end performs the Kronecker product of H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾ and H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) asshown in Formula 37.H _(V) W _(V) ⁽¹⁾ ⊗H _(H) W _(H) ⁽¹⁾(:,l ⁽¹⁾)⊗H _(v) W _(V) ⁽¹⁾(:,k⁽¹⁾)⊗H _(H) W _(H) ⁽¹⁾ =H _(H) W _(H) ⁽¹⁾(:,l ⁽¹⁾)H _(V) W _(V) ⁽¹⁾(:,k⁽¹⁾)H _(V) W _(V) ⁽¹⁾ ⊗H _(H) W _(H) ⁽¹⁾  [Formula 37]

In Formula 37,H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾)⊗H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾ is equal to H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾)H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾)H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾. The reason for this is thatH_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) and H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾) are complex valuesrather than matrices or vectors. Thus, (H_(V)W_(V) ⁽¹⁾)⊗(H_(H)W_(H) ⁽¹⁾)channel may be restored in a manner of dividing a matrix on which theKronecker product is performed by l⁽¹⁾th value of H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾, i.e., (H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾)H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾)).

In the above-mentioned description, the present invention is mainlydescribed with reference to the case that r_(H) ⁽¹⁾ Of H-DM-RS portswhere W_(V) ⁽¹⁾(:,k⁽¹⁾⊗W_(H) ⁽¹⁾ precoding is applied to the wholeantennas in both of the vertical and horizontal directions and (r_(V)⁽¹⁾−1) of V-DM-RS ports where W_(V) ⁽¹⁾(:,˜k⁽¹⁾)⊗W_(H) ⁽¹⁾(:,l⁽¹⁾precoding is applied to the whole antennas in both of the vertical andhorizontal directions are transmitted together. However, in the casethat r_(V) ⁽¹⁾ of V-DM-RS ports where W_(V) ⁽¹⁾⊗W_(H) ⁽¹⁾(:,l⁽¹⁾)precoding is applied to the whole antennas in both of the vertical andhorizontal directions and (r_(H) ⁽¹⁾−1) of H-DM-RS ports where W_(V)⁽¹⁾(:,k⁽¹⁾)⊗W_(H) ⁽¹⁾(:,˜l⁽¹⁾) precoding is applied to the wholeantennas in both of the vertical and horizontal directions aretransmitted together, a channel may be restored in the similar manner.

In particular, when r_(V) ⁽¹⁾ of V-DM-RS ports where W_(V) ⁽¹⁾⊗W_(H)⁽¹⁾(:,l⁽¹⁾) precoding is applied to the whole antennas in both of thevertical and horizontal directions and (r_(H) ⁽¹⁾−1) of H-DM-RS portswhere W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗W_(H) ⁽¹⁾(:,˜l⁽¹⁾) precoding is applied to thewhole antennas in both of the vertical and horizontal directions aretransmitted together, values of H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾)and H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾(:,˜l⁽¹⁾) are measured.

Thus, the receiving end calculates H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾by adding k⁽¹⁾th value of H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) between(l⁽¹⁾−1)th value and l⁽¹⁾th value of H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H)⁽¹⁾(:,˜l⁽¹⁾). Thereafter, the receiving end may perform the Kroneckerproduct of H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾ and H_(V)W_(V)⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) as shown in Formula 38.H _(V) W _(V) ⁽¹⁾ ⊗H _(H) W _(H) ⁽¹⁾(:,l ⁽¹⁾)⊗H _(V) W _(V) ⁽¹⁾(:,k⁽¹⁾)⊗H _(H) W _(H) ⁽¹⁾ =H _(H) W _(H) ⁽¹⁾(:,l ⁽¹⁾)H _(V) W _(V) ⁽¹⁾(:,k⁽¹⁾)H _(V) W _(V) ⁽¹⁾ ⊗H _(H) W _(H) ⁽¹⁾  [Formula 38]

In Formula 38, H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾)⊗H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾)⊗H_(H)W_(H) ⁽¹⁾ is equal to H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾)H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾)H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾ similar to Formula 37. Thereason for this is that H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾) and H_(V)W_(V)⁽¹⁾(:,k⁽¹⁾) are complex values rather than matrices or vectors. Thus,(H_(V)W_(V) ⁽¹⁾)⊗(H_(H)W_(H) ⁽¹⁾)channel may be restored in a manner ofdividing a matrix on which the Kronecker product is performed by k⁽¹⁾thvalue of H_(V)W_(V) ⁽¹⁾⊗H_(H)W_(H) ⁽¹⁾(:,l⁽¹⁾), i.e., (H_(H)W_(H)⁽¹⁾(:,l⁽¹⁾)H_(V)W_(V) ⁽¹⁾(:,k⁽¹⁾)).

Embodiments of the Present Invention with Respect to Multiple ReceivingAntennas

The above-mentioned embodiments are described with reference to the caseof N_(P)=1. According to the following embodiments, the presentinvention can be generalized irrespective of N_(P) by applying itextensively. In this case, precoding is equal to Formula 32. Thus, ifthe precoding is applied to a channel, it may be represented as Formula39 in accordance with Formula 32 and Formula 34.H _(T) ×PM=(H _(V) ⊗H _(H))×└W _(V) ⁽¹⁾ ⊗W _(H) ⁽¹⁾ W _(V) ⁽²⁾ ⊗W _(H)⁽²⁾ . . . W _(V) ^((i)) ⊗W _(H) ^((i)) . . . W _(V) ^((N) ^(P) ⁾ ⊗W _(H)^((N) ^(P) ⁾ ┘=[H _(V) W _(V) ⁽¹⁾ ⊗H _(H) W _(H) ⁽¹⁾ H _(V) W _(V) ⁽²⁾⊗H _(H) W _(H) ⁽²⁾ . . . H _(V) W _(V) ^((i)) ⊗H _(H) W _(H) ^((i)) . .. H _(V) W _(V) ^((N) ^(P) ⁾ ⊗H _(H) W _(H) ^((N) ^(P) ⁾]

According to the 2^(nd) embodiment of the present invention, channelsH_(V)W_(V) ^((i)) and H_(H)W_(H) ^((i)) may be informed the receivingend to reduce overhead of DM-RS. Therefore, the receiving end mayrestore H_(T)×PM using the received channels H_(V)W_(V) ^((i)) andH_(H)W_(H) ^((i)). In the following description, two representativeembodiments related to the 2^(nd) embodiment of the present inventionare explained.

According to the 1^(st) embodiment, for every i, r_(V) ^((i)) of DM-RSports (hereinafter referred to as V-DM-RS) where W_(V) ^((i)) precodingis applied to antennas of a specific column (e.g., 1^(st) column) in thevertical direction and r_(H) ^((i)) of DM-RS ports (hereinafter referredto as H-DM-RS) where w_(H) ^((i)) precoding is applied to antennas of aspecific row (e.g., 1^(st) row) in the horizontal direction may betransmitted together.

Thus, in each receiving antenna, the receiving end performs theKronecker product of channel H_(V)W_(V) ^((i)) estimated through V-DM-RSand channel H_(H)W_(H) ^((i)) estimated through H-DM-RS for each indexi, arranges the results as shown in Formula 36, and may then restore achannel required in demodulation.

In case that either r_(V) ^((i)) or r_(H) ^((i)) is 1, for example, ifr_(V) ^((i)) is 1, only H-DM-RS having r_(H) ^((i)) ports is used. And,if the r_(H) ^((i)) is 1, only V-DM-RS having r_(V) ^((i)) ports isused. In this case, DM-RS applies W_(V) ^((i))⊗W_(H) ^((i)) precoding towhole antennas in both of the vertical and horizontal directions. Ifboth r_(V) ^((i)) and r_(H) ^((i)) are 1, the number of DM-RS portsbecomes 1. In this case, DM-RS applies the W_(V) ^((i))⊗W_(H) ^((i))precoding. Therefore, it is preferred to apply the present invention tothe case that either r_(V) ^((i)) or r_(H) ^((i)) is equal to or greaterthan 2.

Therefore, according the present invention, overhead of DM-RS can bereduced compared to the conventional method of transmitting DM-RS. Inparticular,

$\sum\limits_{i}{r_{V}^{(i)}r_{H}^{(i)}}$of DM-RS ports are required according to the conventional DM-RS whereas

$\sum\limits_{i}\left( {r_{V}^{(i)} \times r_{H}^{(i)}} \right)$of DM-RS ports are required according to the present invention.Therefore, the amount of overhead can be reduced.

According to the 2nd embodiment, for each i, V-DM-RS ports correspondingto antennas in the vertical direction and H-DM-RS ports corresponding toantennas in the horizontal direction among whole antennas in both of thevertical and horizontal directions are simultaneously transmitted.Moreover, DM-RS ports for antennas included in both of the antennas inthe vertical direction and the antennas in the horizontal direction maybe configured to be transmitted once.

For instance, r_(H) ^((i)) of H-DM-RS ports where W_(v)^((i))(:,k^((i)))⊗W_(H) ^((i)) precoding is applied to the wholeantennas in both of the vertical and horizontal directions and (r_(V)^((i))−1) of V-DM-RS ports where W_(V) ^((i))(:,˜k^((i)))⊗W_(H)^((i))(:,l^((i))) precoding is applied to the whole antennas in both ofthe vertical and horizontal directions may be transmitted together. Asanother example, r_(V) ^((i)) of V-DM-RS ports where W_(V) ^((i))⊗W_(H)^((i))(:,l^((i))) precoding is applied to the whole antennas in both ofthe vertical and horizontal directions and (r_(H) ^((i))−1) of H-DM-RSports where W_(V) ^((i))(:,k^((i)))⊗W_(H) ^((i))(:,˜l^((i))) precodingis applied to the whole antennas in both of the vertical and horizontaldirections may be transmitted together.

According to the 2^(nd) embodiment, the total number of DM-RS portsbecomes‘

${{\sum\limits_{i}r_{H}^{(i)}} + r_{V}^{(i)} - 1},$Compared to

$\sum\limits_{i}{r_{V}^{(i)}r_{H}^{(i)}}$of DM-RS ports in the related art, the total number of DM-RS ports isreduced. Moreover, compared to‘

${\sum\limits_{i}\left( {r_{V}^{(i)} \times r_{H}^{(i)}} \right)},$of DM-RS ports in the 1^(st) embodiment, one DM-RS port can be reduced.In the 2^(nd) embodiment, indices k^((i)) and l^((i)) may be previouslydefined between the transmitting and receiving ends or be configuredthrough RRC signaling. The receiving end may restore a channel using achannel estimated through received V-DM-RS and H-DM RS in each receivingantenna.

If r_(H) ^((i)) of H-DM-RS ports where W_(V) ^((i))(:,k^((i)))⊗W_(H)^((i)) precoding is applied to the whole antennas in both of thevertical and horizontal directions and (r_(V) ^((i))−1) of V-DM-RS portswhere (W_(V) ^((i))(:,˜k^((i)))⊗W_(H) ^((i))(:,l^((i))) precoding isapplied to the whole antennas in both of the vertical and horizontaldirections may be transmitted together according to the 2nd embodiment,the receiving end measure values of H_(V)W_(V)^((i)))(:,k^((i)))⊗H_(H)W_(H) ^((i)) and H_(V)W_(V)^((i))(:,˜k^((i)))⊗H_(H)W_(H) ^((i))(:,l^((i))) for each index i. And,the receiving end calculates H_(V)W_(V) ^((i))⊗H_(H)W_(H)^((i))(:,l^((i))) by adding l^((i))th value of H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)) between (k^((i))−1)^(th) value andk^((i))th value of H_(V)W_(V) ^((i))(:,˜k^((i)))⊗H_(H)W_(H)^((i))(:,l^((i))). Subsequently, the receiving end performs theKronecker product of H_(V)W_(V) ^((i))(:,k)⊗H_(H)W_(H) ^((i)) andH_(V)W_(V) ^((i))⊗H_(H)W_(H) ^((i))(:,l^((i)) ) as shown in Formula 40.H _(V) W _(V) ^((i)) ⊗H _(H) W _(H) ^((i))(:,l ^((i)))⊗H _(V) W _(V)^((i))(:,k ^((i)))⊗H _(H) W _(H) ^((i)) =H _(H) W _(H) ^((i))(:,l^((i)))H _(V) W _(V) ^((i))(:,k ^((i)))H _(V) W _(V) ^((i)) ⊗H _(H) W_(H) ^((i))  [Formula 40]

In Formula 40, H_(V)W_(V) ^((i))⊗H_(H)W_(H) ^((i))(:,l^((i)))⊗H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)) is equal to H_(H)W_(H)^((i))(:,l^((i)))H_(V)W_(V) ^((i))(:,k^((i)))H_(V)W_(V)^((i))⊗H_(H)W_(H) ^((i)). The reason for this is that H_(H)W_(H)^((i))(:,l^((i))) and H_(V)W_(V) ^((i))(:,k^((i))) are complex valuesrather than matrices or vectors. Thus, (H_(v)W_(V) ^((i)))⊗(H_(H)W_(H)^((i))) channel is restored by dividing a matrix on which the Kroneckerproduct is performed by l^((i))th value of H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)), i.e., (H_(H)W_(H)^((i))(:,l^((i)))H_(V)W_(V) ^((i))(:,k^((i)))). Thereafter, the channelrequired in demodulation can be restored by arranging them according toFormula 39.

In the above-mentioned description, the present invention is mainlydescribed with reference to the case that r_(H) ^((i)) of H-DM-RS portswhere W_(V) ^((i))(:,k^((i)))⊗W_(H) ^((i)) precoding is applied to thewhole antennas in both of the vertical and horizontal directions and(r_(V) ^((i))−1) of V-DM-RS ports where W_(V) ^((i))(:,˜k^((i)))⊗W_(H)^((i))(:,l^((i))) precoding is applied to the whole antennas in both ofthe vertical and horizontal directions are transmitted together.However, in the case that r_(V) ^((i)) of V-DM-RS ports where W_(V)^((i))⊗W_(H) ^((i))(:,l^((i))) precoding is applied to the wholeantennas in both of the vertical and horizontal directions and (r_(H)^((i))−1) of H-DM-RS ports where W_(V) ^((i))(:,k^((i)))⊗W_(H)^((i))(:,˜l^((i))) precoding is applied to the whole antennas in both ofthe vertical and horizontal directions are transmitted together, achannel may be restored in the similar manner.

In particular, when r_(V) ^((i)) of V-DM-RS ports where W_(V)^((i))⊗W_(H) ^((i))(:,l^((i))) precoding is applied to the wholeantennas in both of the vertical and horizontal directions and (r_(H)^((i))−1) of H-DM-RS ports where W_(V) ^((i))(:,k^((i)))⊗W_(H)^((i))(:,˜l^((i))) precoding is applied to the whole antennas in both ofthe vertical and horizontal directions are transmitted together, valuesof H_(V)W_(V) ^((i))⊗H_(H)W_(H) ^((i))(:,l^((i))) and H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i))(:,˜l^((i))) are measured for each i.

Thus, the receiving end calculates H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)) by adding k^((i))th value ofH_(V)W_(V) ^((i))⊗H_(H)W_(H) ^((i))(:,l^((i))) between (l^((i))−1)^(th)value and l^((i))th value of H_(V)W_(V) ^((i))(:,k^((i)))⊗H_(H)W_(H)^((i))(:,˜l^((i))). Thereafter, the receiving end may perform theKronecker product of H_(V)W_(V) ^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)) andH_(V)W_(V) ^((i))⊗H_(H)W_(H) ^((i))(:,l^((i))) as shown in Formula 41.H _(V) W _(V) ^((i)) ⊗H _(H) W _(H) ^((i))(:,l ^((i)))⊗H _(V) W _(V)^((i))(:,k ^((i)))⊗H _(H) W _(H) ^((i)) =H _(H) W _(H) ^((i))(:,l^((i)))H _(V) W _(V) ^((i))(:,k ^((i)))H _(V) W _(V) ^((i)) ⊗H _(H) W_(H) ^((i))  [Formula 41]

In Formula 41, H_(V)W_(V) ^((i))⊗H_(H)W_(H) ^((i))(:,l^((i)))⊗H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)) is equal to H_(H)W_(H)^((i))(:,l^((i)))H_(V)W_(V) ^((i))(:,k^((i)))H_(V)W_(V)^((i))⊗H_(H)W_(H) ^((i)). The reason for this is that H_(H)W_(H)^((i))(:,l^((i))) and H_(V)W_(V) ^((i))(:,k^((i))) are complex valuesrather than matrices or vectors. Thus, (H_(V)W_(V) ^((i)))⊗H_(H)W_(H)^((i))) channel is restored by dividing a matrix on which the Kroneckerproduct is performed by l^((i))th value of H_(V)W_(V)^((i))(:,k^((i)))⊗H_(H)W_(H) ^((i)), i.e., (H_(H)W_(H)^((i))(:,l^((i)))H_(V)W_(V) ^((i))(:,k^((i)))). Thereafter, the channelrequired in demodulation can be restored by arranging them according toFormula 39.

The 1^(st) and 2^(nd) embodiments of the present invention are mainlyexplained with reference to the case that DM-RSs are transmitted in theforms of H-DM-RS and V-DM-RS depending on index i. However, a receivingend may transmit one DM-RS by combining them. Alternatively, thereceiving end may create and use H-DM-RS pattern and V-DM-RS pattern forN_(P) size of H-DM-RS and N_(P) size of V-DM-RS, respectively.

Moreover, according to the present invention, since the receiving endrestores a channel using the Kronecker product after estimation, new DCImay be defined. In this case, for the new DCI, the number of H-DM-RSports and the number of V-DM-RS ports may be defined in each index i.Alternatively, r_(V) ^((i)) and r_(H) ^((i)) may be defined in eachindex i.

Furthermore, the new DCI may include a bit (e.g., 0/1 index) forinforming the receiving end of whether to apply theembodiment/rule/configuration proposed in the present invention or touse DM-RS ports as many as the number of layers in the same manner asthe legacy system.

In addition, if one DM-RS is transmitted by combining the N_(P) size ofH-DM-RS and the N_(P) of V-DM-RS, the receiving end needs to know whichports of DM-RS corresponds to H-DM-RS port of index i and V-DM-RS portof index i, respectively. Therefore, information on H-DM-RS/V-DM-RS maybe explicitly indicated through the new DCI or pre-defined between atransmitting end and the receiving end. Alternatively, a relatedconfiguration may be informed through RRC.

FIG. 26 is a diagram for configurations of a base station device and auser equipment device according to the present invention.

Referring to FIG. 26, a base station device 2610 according to thepresent invention may include a receiving module 2611, a transmittingmodule 2612, a processor 2613, a memory 2614 and a plurality of antennas2615. In this case, a plurality of the antennas 2615 may mean a basestation device that supports MIMO transmission and reception. Thereceiving module 2611 may receive various signals, data, information andthe like in uplink from a user equipment. The transmitting module 2612may transmit various signals, data, information and the like in downlinkto the user equipment. Moreover, the processor 2613 may be configured tocontrol overall operations of the base station device 2610.

The base station device 2610 according to one embodiment of the presentinvention may be configured to transmit a DL signal. And, the memory2614 of the base station device 2610 may store codebook includingprecoding matrixes. The processor 2613 of the base station device 2610may be configured to receive 1^(st) PMI (precoding matrix indicator) and2^(nd) PMI from the user equipment through the receiving module 2611.The processor 2613 may be configured to determine a 1^(st) matrix (W1)from 1^(st) codebook including the precoding matrixes indicated by the1^(st) PMI and determine a 2^(nd) matrix (W2) from 2^(nd) codebookincluding the precoding matrixes indicated by the 2^(nd) PMI. Theprocessor 2613 may be configured to determine a precoding matrix (W)based on the 1^(st) matrix (W1) and the 2^(nd) matrix (W2). Theprocessor 2613 may be configured to perform precoding on at least onelayer, to which the DL signal is mapped, using the determined precodingmatrix (W). The processor 2613 may be configured to transmit theprecoded signal to the user equipment through the transmitting module2612. In this case, each of the precoding matrixes included in the1^(st) codebook includes a block diagonal matrix. And, one block mayhave a form multiplied by a prescribed phase value, which is differentfrom a different block.

The processor 2613 of the base station device 2610 performs a functionof processing information received by the base station device 2610,information to be externally transmitted and the like. The memory 2614may store the processed information and the like for prescribed durationand be substituted with such a component as a buffer (not shown in thedrawing) or the like.

Referring to FIG. 26, a user equipment device 2620 according to thepresent invention may include a receiving module 2621, a transmittingmodule 2622, a processor 2623, a memory 2624 and a plurality of antennas2625. In this case, a plurality of the antennas 2625 may mean a userequipment device that supports MIMO transmission and reception. Thereceiving module 2621 may receive various signals, data, information andthe like in downlink from a base station. The transmitting module 2622may transmit various signals, data, information and the like in uplinkto the base station. Moreover, the processor 2623 may be configured tocontrol overall operations of the user equipment device 2620.

The user equipment device 2620 according to one embodiment of thepresent invention may be configured to receive and process a DL signal.And, the memory 2624 of the user equipment device 2620 may storecodebook including precoding matrixes. The processor 2623 of the userequipment device 2620 may be configured to transmit 1^(st) PMI(precoding matrix indicator) and 2^(nd) PMI to the base station throughthe transmitting module 2622. The processor 2623 may be configured toreceive the DL signal through the receiving module 2621. In this case,the DL signal received by the user equipment corresponds to the DLsignal precoded by the base station using the precoding matrix (W). Inparticular, the precoding may be performed by the base station on atleast one layer to which the DL signal is mapped. In this case, theprecoding matrix (W) may be determined based on the 1^(st) matrix (W1)determined from the 1^(st) codebook including the precoding matrixesindicated by the 1^(st) PMI and the 2^(nd) matrix (W2) determined fromthe 2^(nd) codebook including the precoding matrixes indicated by the2^(nd) PMI. The processor 2613 may be configured to determine aprecoding matrix (W) based on the 1^(st) matrix (W1) and the 2^(nd)matrix (W2). The processor 2613 may be configured to process thereceived DL signal using the determined precoding matrix (W). In thiscase, each of the precoding matrixes included in the 1^(st) codebookincludes a block diagonal matrix. And, one block may have a formmultiplied by a prescribed phase value, which is different from adifferent block.

The processor 2623 of the user equipment device 2620 performs a functionof processing information received by the user equipment device 2620,information to be externally transmitted and the like. The memory 2624may store the processed information and the like for prescribed durationand be substituted with such a component as a buffer (not shown in thedrawing) or the like.

The detailed configurations of the base station device and the userequipment device mentioned in the above description may be implementedin a manner that the matters of various embodiments of the presentinvention mentioned in the foregoing description are independentlyapplied or that at least two embodiments of the present invention aresimultaneously applied. And, redundant contents may be omitted forclarity.

In the description with reference to FIG. 26, the description of thebase station device 2610 may be identically applicable to a relay deviceas a downlink transmission entity or an uplink reception entity. And,the description of the user equipment device 2620 may be identicallyapplicable to a relay device as a downlink reception entity or an uplinktransmission entity.

The embodiments of the present invention may be implemented usingvarious means. For instance, the embodiments of the present inventionmay be implemented using hardware, firmware, software and/or anycombinations thereof.

In case of the implementation by hardware, a method according to eachembodiment of the present invention may be implemented by at least oneselected from the group consisting of ASICs (application specificintegrated circuits), DSPs (digital signal processors), DSPDs (digitalsignal processing devices), PLDs (programmable logic devices), FPGAs(field programmable gate arrays), processor, controller,microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, a methodaccording to each embodiment of the present invention can be implementedby modules, procedures, and/or functions for performing theabove-explained functions or operations. Software code may be stored ina memory unit and may be then drivable by a processor. The memory unitmay be provided within or outside the processor to exchange data withthe processor through the various means known to the public.

As mentioned in the foregoing description, the detailed descriptions forthe preferred embodiments of the present invention are provided to beimplemented by those skilled in the art. While the present invention hasbeen described and illustrated herein with reference to the preferredembodiments thereof, it will be apparent to those skilled in the artthat various modifications and variations can be made therein withoutdeparting from the spirit and scope of the invention. For instance, therespective configurations disclosed in the aforesaid embodiments of thepresent invention can be used by those skilled in the art in a manner ofbeing combined with one another. Therefore, the present invention isnon-limited by the embodiments disclosed herein but intends to give abroadest scope matching the principles and new features disclosedherein.

The present invention may be embodied in other specific forms withoutdeparting from the spirit and essential characteristics of theinvention. Thus, the above embodiments should be considered in allrespects as exemplary and not restrictive. The scope of the presentinvention should be determined by reasonable interpretation of theappended claims and the present invention covers the modifications andvariations of this invention that come within the scope of the appendedclaims and their equivalents. The present invention is non-limited bythe embodiments disclosed herein but intends to give a broadest scopematching the principles and new features disclosed herein. And, it isapparently understandable that an embodiment is configured by combiningclaims failing to have relation of explicit citation in the appendedclaims together or can be included as new claims by amendment afterfiling an application.

INDUSTRIAL APPLICABILITY

Although a method and apparatus for transmitting a reference signal in awireless communication system supporting multiple antennas are mainlydescribed with reference to the examples of applying to 3GPP LTE system,as mentioned in the foregoing description, the present invention isapplicable to various kinds of wireless communication systems as well asto the 3GPP LTE system.

What is claimed is:
 1. A method of transmitting a reference signal by atransmitting device in a wireless communication system supportingmultiple antennas, the method comprising: combining a horizontaldemodulation reference signal (H-DM-RS) having a first pattern inhorizontal antenna domains and a vertical demodulation reference signal(V-DM-RS) having a second pattern in vertical antenna domains and into aspecific reference signal, to adjust a main lobe of the multipleantennas according to a position of a receiving device; and transmittingthe specific reference signal to the receiving device, to receive achannel state information (CSI) of the receiving device, wherein aspecific antenna domain of the horizontal antenna domains in associationwith the first pattern is overlapped on the vertical antenna domainsbased on the second pattern, and wherein the V-DM-RS is generated basedon the vertical antenna domains except the specific antenna domain. 2.The method of claim 1, wherein the H-DM-RS and the V-DM-RS are used bythe receiving device to restore entire channels of the multiple antennasthrough a Kronecker product.
 3. The method of claim 1, wherein thespecific antenna domain is configured through radio resource control(RRC) signaling.
 4. The method of claim 1, further comprising:transmitting downlink control information (DCI) comprising informationon the horizontal antenna domains and information on the verticalantenna domains.
 5. The method of claim 4, wherein the DCI furthercomprises a bit index indicating a specific reference signal scheme inthe transmitting device.
 6. In transmitting a reference signal in awireless communication system supporting multiple antennas, atransmitting device comprising: a transmitter; and a processor, whereinthe processor is configured to: combine a horizontal demodulationreference signal (H-DM-RS) having a first pattern in horizontal antennadomains and a vertical demodulation reference signal (V-DM-RS) having asecond pattern in vertical antenna domains and into a specific referencesignal, to adjust a main lobe of the multiple antennas according to aposition of a receiving device, and control the transmitter to transmitthe specific reference signal to a receiving device, to receive achannel state information (CSI) of the receiving device, wherein aspecific antenna domain of the horizontal antenna domains in associationwith the first pattern is overlapped on the vertical antenna domainsbased on the second pattern, and wherein the V-DM-RS is generated basedon the vertical antenna domains except the specific antenna domain.